1. Click on a state
   
2. click on the arrow above the state info interface (as indicated in the photo
   
3. choose what building you wanna build
   
4. Check how much money you have
   
5. Choose the quantity and click "invest" and see the Investment number go up

You will get weekly 6% return on investment
In my case: I have 565.184b$ in investments. That means, that I get earn 0.644b a week.

The mechanic can be useful in helpig to save yourself from dept (on the long run).
AI and other players can send you inv. offers. You have to pay for a small part of the project.

This may help you out if you're a small nation.
Be aware, that every single investment increases influence over you.
Some countries can and will decline your offer, due to ideology of financial reasons (Like N Korea. These suckers have never accepted my money I hate them so much).

Thank you for clicking this guide
I'm glad, that I could help

Comment if you have questions and leave a like or award for my time XD