(and her friends) were originally on "musical.ly" creating and sharing little videos (many could be considered like karaoke types of things). But what happened was that TikTok bought that app/service and literally brought who knows how many over into their platform to continue doing the same thing.
She seems to use it like you see DU posting memes (including animated gifs) where some of those TikTok shorts express some kind of analogous "reaction-via-a-video-short", for some situation that they might want to comment about. When she texts me those clips, since I have no TikTok account, most don't open and demand I login. A few will open, but I'm not sure of how the app works and whether that is because those shorts are "public" or what. 
I think the worst of it is probably shared among that high school age - notably those "TikTok Challenges" crap. I know she has said that some of the idiot boys in her school did one of those challenges where they trash a bathroom in the school. This is a suburban school to boot.
I expect the users of it outside of professional entertainers who are expected to have some presence on "social media" and those who choose to use the internet and social media to make money (whether on that or YouTube, or Instagram), are mostly that younger generation (probably heavy on the GenZ and young millennials). It's just like how when Facebook came out, it was targeted to students in college but picked up many boomers who used it to make contact with old friends and keep in touch with far-flung family (as an alternative to email), that caused the younger ones to flee to MySpace. And when that eventually fell apart (although I saw it was sort of rebooted), it was replaced by a variety of things including SnapChat, another app popular among the young crowd.
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