Facebook is for friends and family sharing bits of their lives, some nastiness, some of the worst politics of right and left. Instagram is similar to Facebook but picture oriented and doesn't really have politics so is nicer.
Twitter is for nasty mobs arguing over minutia - it's responsible for the worst of left wing politics ( right wing stuff doesn't tend to take hold ).
I'm trying to get less Twitter in my life. I've uninstalled it from my phone.
TikTok is different in that 1 minute videos are shared, (automatically?) categorised and watched, one after another, by people whose account is deemed suitable for the video. You can also follow people who you find interesting. For example I see occasional videos from a British doctor talking about weird medical facts, etc. I see historical facts ( a chap who collects WW1 memorabilia does interesting videos ).
The videos are one minute long and, once you've "trained" it by skipping tedius stuff like the dancing, it's mostly interesting. If you like the thing you watch it, if it bores you you skip it or press "not interested" and it learns to give you interesting things.
It's the first social media platform that pretty much always makes me happier when I use it. The rest are either often nasty or boring.
My guess is that TikTok "won" because of the algorithm of which video to show you works much better than Vines ever did ( I never used Vine and I only installed TikTok out of curiosity but I'm glad I did ).
It's not competing with YouTube because it decides what to show you rather than the other way around so it's much more passive.