YES, you can develop, support open source/Free Software (FOSS) projects as an unpaid hobby, just for fun.
YES, you can develop and support #FOSS projects as (big or small) part of your paid job or as freelancer.
YES, you can develop and support FOSS projects by asking for donations.
And yes, all of these approaches can exist at the same time. YOU decide. FOSS is about freedom, not about money or not.
And NO, you owe no-one an explanation or justification on why you do #FOSS the way you do.