-do their employers recognize their effort and give hours to the side projects?
-they mostly contractors so can arrange their time accordingly?
-donations?
-or is it mostly a hobby done out of passion?
No payment, not a hobby, very little passion, doing it because the public needs it. (And possibly got started because, damn it, somebody had to write it / take over maintenance.)
Further, as a contributor of some now popular projects with one other maintainer, I can say that I mostly do it for fun. My current employer gives zero-fucks about it but if a new employer were to use it to woo me away, I would jump in a heartbeat.