The design is very very different. Youtube's algorithm isn't designed for short clips, and you have to load a new page every time you want a new clip. Tiktok has an automatic, very-fast scroll. It's like Youtube's Eharmony and Tiktok is Tinder.
wow this is a cool song, I'll dance to it
tiktok: thousands discover your video by searching for dances for that specific song
youtube: copyright violation, removed