Do you not sponsor projects that you get value out of?

I'm not saying you have to, but you asked how they get compensated and there's nothing stopping you from giving them money.

It's easy to forget that you get a lot of value out of something and not give back. If you end up getting a good paying job with your programming experience just buy your favorite projects "a beer" one a month, or once a year. God knows it's better spent there all the subscriptions we have like Netflix or Spotify. Cheaper too.

Also, if the projects are big enough you can usually get tax credit. If you work at a decently sized company they also usually do some charity matching.