Here be intro.

Some wiki links (ctrl + left click OR middle click to open in new tab instead of new window):
Items - https://synthetik.fandom.com/wiki/Items
Weapons - https://synthetik.fandom.com/wiki/Weapons
Chests - https://synthetik.fandom.com/wiki/Chests
Curses - https://synthetik.fandom.com/wiki/Curses
Modules - https://synthetik.fandom.com/wiki/Memory_Module_Reference
Attachments - https://synthetik.fandom.com/wiki/Attachments
Variants - https://synthetik.fandom.com/wiki/Variants
Artifacts - https://synthetik.fandom.com/wiki/Artifacts

Also, DO NOT BUY "Variant Kit 2" from the research shop. As of now (2023 01 21) both variants that unlocks are pretty bad.

If you did already, you can BACK UP YOUR SAVE TO A SAFE PLACE and then change the line
back to
so it's locked again.

Here be missing recycling info.

Rosarius: +100 max shield, non-scaling. While kept unrecycled: +20 Luck.

Standard. Has to be beat at >=140% difficulty to finish class challenge and at >=200% as a part of insanity class challenge.

Start with mildly boosted android stats and greatly increased terrorlevel (i.e. more/harder enemies, but also better money). Intended to be a harder mode with a faster pace than classic. Has to be beat at >=140% difficulty as a part of insanity class challenge.
Costs 100 data to start a run (paid by the host), you have to beat Last Defender at least once to unlock it.

Also known as HA mode.
All damage dealt and received is 300% of normal - damage absorb is made to be -200 to start and damage output is made to be 300. This means that the curses from Zir shrines are mostly not worth taking in this mode. Everything is a matter of life and death. High alpha damage and/or good damage avoidance skills are a must - you can often die to single shots.
Costs 100 data to start a run (paid by the host), you have to beat Last Defender at least twice to unlock it. Has to be beat at >=140% difficulty as a part of insanity class challenge.

Also known as the boring mode.
The player moves slower, with next to no shield. Start with slightly higher max health, but the healing starts at half power. Lowered firerate, increased heat generation and recoil, far longer dash cooldown. Humanoid enemies have greater health, but player deals more damage to bosses. Start with a few random items from a small pool (which will consume any secondary item at the spawn if you don't drop it beforehand) as well as the selected class' core item. Wiki states that you "start with 2000 credits, and all items have 200% power when picked up. However, normal item crates do not spawn.", but I managed to find item crates sometimes (rarely, though). "A slow and methodical approach is required to succeed unharmed."
Costs 100 data to start a run (paid by the host), you have to beat Last Defender at least twice to unlock it. Has to be beat at >=140% difficulty as a part of insanity class challenge.

Start with a random weapon, and two random items which replace the player's selected starting items, excluding their core item. The player also gains 30% more experience while playing in daily mode. A quick run to gain experience and data with little consequence.
Costs 0 data to begin, can be played once per day.

You can cheat a little by dropping your items - fast enough so they don't get consumed in the exchange - and picking them up again after the daily run items show up.

Also, unmentioned, there was some small bonus to some stats during daily run, I think it was ammo pickup or something like that.

Costs 125 data to begin. "Gain reduced data", but get a random item, weapon or credits at the start. Can be bought next to an actual mode, but only by the client player (not the host), - host's UI will not reflect that before starting the game, but in-game the selected mode will be visible and the starting bonus will be given.

Item/weapon chests which look like made of wood have lower level stuff inside than the ones made with metal. Which doesn't strictly translate to "worse", mind you.

Item/weapon chests with black and white squares (not unlike a chessboard) - timed chests. Go for them, but do not aggravate the whole level just to get there - you'll die. They are supposed to have a little bit better stuff inside, but it's still not worth throwing your life and/or run away.

Item/weapon chests with black and yellow diagonal bars - "alarmed" chests. When opened, they alert all nearby enemies to the player's location, summon an elite enemy, and raise the overall terror level by 15 upon opening. Not not worth it - more enemies means more money.

Mystery chests (with question marks on them) - they can contain an item, weapon, weapon upgrade kit, Armageddon shard, keycard (inc. Multi-Pass), core module, ammo, plating, or vial charge. May also require credits to open. Has a unique loot table with items that can not be gained otherwise.

Grey-blue "UPG" chests - grants a 10% increase in damage to the currently equipped weapon. Try to use it on end-game weapons you didn't upgrade yet, it's supposedly stronger then.

Yellow "UPG" chests - similar to the Android Upgrade Module, provides a choice of three upgrades to the current character's core stats and abilities.

Rare chests:

• There is only one type of chest that can be picked up by both players - a golden chest with black top with a red "K" written on it. Gives two high powered random body upgrades (permanent stat boosts) as well as 5000 experience; this is the only chest that can be opened by both players in co-op.
  
• Orange-red "OOOAF" chest - gives a choice of three randomly-chosen class modules, chosen from all classes.
  
• Green "OOOAF" chest - grants a full artifact, to be chosen from a selection of 3 to 4
  
• Yellow "OOOAF" chest - unlocks all weapon ammo types for all weapons.
  
• Metal chest with black/yellow top with CRX written on it - provides a large amount of data and Credits. Only found by killing a Treasure Roomba before it escapes.

Other drops/"powerups" - ammo (yellow-black rectangles), ammo regen (five green vertical bullets), blue kits, vials, etc. are visible and pick-upable ONLY by the player who sees them. The only exception to this are items dropped by either player, which will look like either modules or "white kits".

Curses in the game are many things, but some of them you don't have to worry about.

"Cursed" items are just stronger items. No downside to using them whatsoever.

Cursed chests - Damages player down to 100 hp (!), permanent 50 less shield, temporarily lose 100 max shield (for a total of 150). 10%-15% more damage taken. Which isn't that bad by itself e.g. in HA mode. Negative status effects last longer. Terror Level increased. Can stack. All stacks are removed on a boss kill. Yields a high level weapon/item, whatever that means.

Cursed weapons - will harm the user when used, with a % chance (10%?).

Cursed flesh gained (up to two times) when you die in a co-op game (doesn't matter who is the game host). Reduces shields temporarily, and lose 50 max shield permanently. Increases damage taken percentage. All negative and positive buffs last longer. Terror Level increased.
Do not confuse with curse of the flesh gained at a Zir shrine.

Zir shrine stuff:

• Prayer - the only non-curse available. Chance to lift a curse and/or grant a single buff lasting usually 1 minute and/or grant Data.
  
• Curse of the flesh - decreases maximum health by 400, but increases currency gain by 40. Do not confuse with Cursed Flesh.
  
• Curse of the lethargic - decreases dodge by 15 and reload speed by 10%, increases luck by 40. Can cause dodge to go negative.
  
• Curse of the wicked - decreases damage absorb by 40, increases damage output by 30. Not worth it in HA mode (where you are at -200 and +300, respectively, so -40/+30 is a bad deal AND a drop in a bucket)
  
• Curse of the ignorant - currency gain stat reduced by 50, increases damage absorb by 30, thus best used just before the LD. Can stack. Can cause currency to become a negative value if used multiple times.
  
• Curse of the partisan - reduces damage absorb stat by 50, increased maximum shield by 250 and removes curses gained from chests or co-op deaths. One of the few ways of clearing the curses; I wouldn't go this route if there was any other way.
  
• Tribute of flesh - decreases maximum health by 200, refills health and ammo, grants 2 weapon upgrade kits and 2 vial charges.
  
• Tribute of copper - consumes up to 666 credits, refills health and ammo, consumes 100 data from the lab in addition if the tribute is not at least 666 credits.

Curses gained at Zir shrines are consensual sacrifices, so they cannot be taken off. It is advisable to use a Zir shrine only after clearing the level if you are not in a pinch.

There are only three means of cleaning curses:

• Killing a boss (even if the player is dead when the boss dies)
  
• Achieving the maximum number of stacks with the Devil's Dice item
  
• Praying at Sacrifice Shrines

Items seem to come in quite a few qualities, but these DO NOT MATTER overall.

Even the "cursed" items are perfectly safe. Well, as safe as the items come, but "Standard" and "Cursed" - and all other item quality descriptors differ (only?) in starting item power.

Quick lookup instead of venturing into wiki below. Data from wiki:

* Revised: 100% Power, 25% Faster Cooldown. Replaces Damaged quality items when playing as an Engineer at Prestige 1 or higher.

Also, some item qualities are more chance based than others:

• Divine = 1% chance * luck
  
• '''????''' = 0.1% chance * luck

I am pretty sure (not 100% certain, though), that item "qualities" do NOT influence neither power percentage gained from upgrades nor the upgrade cost itself, but I never cared enough for this to write it down and check, so here we are.

Unsorted tips from various sources.

• Try to use the upgrade kits on the boss levels only. If no great end-game weapon candidate is available, skip it till the next boss. Why? Because using them in boss rooms gives you chance for better/more rare upgrade choices.
  
• Upgrade upgrade kits. Do not use the last one so the bonus percentage stays at higher level.
  
• I can't recall now, but there was some shenanigans to be done with the suitcase item for engineer. Taking two of them or something? Transmuting it into something else?
  
• Module core starting item - there was something special about that as well, but I cannot recall what exactly now.