Characters in Friends all have arcs and evolve. At the end of the show, all characters are better off than at the start. Joey starts out very poor and by S6E18 is $3400 in debt to Chandler.
In Season 2, Joey gets a glimpse at being financially well off and over-spends. Between S3 and S7, he has to take small jobs to make ends meet. He teaches soap opera ...
I know we are following tv story rules. But the writer's themselves don't follow them. So why bother.
We rarely see anyone working, but they are living comfortably in New York. So I don't really think it matters how one has money in a sitcom.