The main thing they all have in common is that none of them have a single source of income. Ultimately, the zine author isn't living just off their zines (even if the zine is the main thing). They also likely have three or four other things going on, and they probably all interface with each other to support the person. Maybe one thing makes profitable use of the output of, or waste product of, another, for instance. Maybe the different things tickle the same artistic bone, or use the same skills or equipment. Maybe they are economically different enough so that when times are weak for one, they're strong for another. That sort of thing.
As a concrete example, I have a friend who publishes zines, but also sells merch (some made by others on commission) that uses artwork from the zines, sells small paintings in local indie music stores, coffee shops, book stores, etc. and does commission work.
In straight business terms, it's really just the concepts of diversification and synergy applied in a way that seems unusual. That it seems unusual is interesting to me, because it's not, really. It's very old-school.