For positions like café worker, bartender, gym trainer, the game will pull townies from the available pool. Some of the premade townies have jobs -- Liberty Lee is an astronaut, Travis Scott a tech guru, etc. Homeless townies usually don't have fixed jobs. Several patches ago, the game was adjusted so that everyone was given a job, but the people who played rotationally didn't like that. They would leave a sim with one job and come back to find that sim in a different job -- or unemployed.
So the answer to your question is that they might get jobs on their own. I don't know about gallery sims or mods. As for sims you place in your game -- you can set them up in one file, have them get a job, even cheat them through a few promotions, and then save them to your library. If you then place them as townies in a new game, they will keep their skills and jobs. A word of advice here -- if you don't want them culled, it's a good idea to place them in a house rather than let them be homeless.