Usually they are supported by donations, therefore they don't need to sell their user's data or their software.

The same way you can make model trains and not make money.

The Stardew Valley gambit - quit programming and take up subsistence farming.

Good luck. Farming is a hard business if you want to make money. And you'll need money for electricity, fuel, medicine, etc.

I have an employer who pays me to do thing X. And they don’t care that I also work on thing Y a little bit.

I think there’s lots of software written by people who have jobs and code because it’s fun.

For example, Linus Torvalds made Subsurface [0] as open source. He had a job while he made this. He didn’t get paid for it directly, but it’s not like paying him extra would make it better.

[0] https://subsurface-divelog.org/