The topic would probably impact how fast the growth is, but not the growth in general. A lot of very niche creators find better success than more general people.

As for finding trends, there are accounts I follow that post about trends before they're big. But, I mostly find trends just by being an avid user of the app in general. If I see a few people start to do a video format and click on the dong or hashtag associated with that idea and see minimal videos (under 2,000) for the idea, then I know it's not big yet.