Rich is relative - I've written two books for a publisher and a co-authorship of a self-published book. In both cases, it was nowhere enough to recover the time spent on it. However, seeing the sales figures, seeing your work cited in publications and the reactions you get from people when they say "Oh you wrote that?" - that is another feeling by itself.

You have to remember that a lot of these books are written as groundwork for future plans - far easier to establish credentials saying "yeah I've written that book, you may have heard of it".

Probably? Check out https://news.ycombinator.com/item?id=23818859 and https://jvns.ca/blog/2019/10/01/zine-revenue-2019/

It also depends on what's your goal. In my case, I was just looking for a way to pay my bills and not having to go back to a job again. My monthly expenses where I live is around $150 and I'm earning about $200 per month as a self-published author. That's more than I need and I'm grateful for that.