"and to never make it online."

So then of course, you also cannot sell your work, as those might put it online. And you cannot show your art to big crowds, as some will make pictures and put it online. So ... you can become a literal underground artists, where only some may see your work. I think only some will like that.

But I actually disagree, there are plenty of ways to be an artist now - but most should probably think about including AI as a tool, if they still want to make money. But with the exception of some superstars, most artists are famously low on money - and AI did not introduce this. (all the professional artists I know, those who went to art school - do not make their income with their art)

GP almost certainly mean "make physical art." Pictures of that can get online, but it's not the real thing.

Can you elaborate on how they supplement their income?

Every other source of income? So other, art-unrelated jobs.