New to this - what is difference between tiktok and youtube?

rough version: TikTok pushes short videos to people who don't follow you, so a post can blow up then die within days. YouTube is also a search engine, so a decent video can keep getting found for months or years. That two weeks then flat thing you saw is very typical of TikTok. Each video gets its burst and that's it, so it rewards posting often. YouTube rewards fewer, better videos built around things people search for. Small scale, pick the one where your audience actually spends time rather than doing both badly.