Why do I remember my dreams after watching TikTok videos before bed?

It's because your brain is still in active mode when you go to bed - TikTok is super stimulating, it's full of content that changes fast, and it keeps your attention at 100%. When you sleep after that, you go through a phase of light sleep before really getting into it, and during that period your brain holds onto dreams better. If you do something calmer before bed instead, you wind down gradually and you forget your dreams like usual.