No, the music overlay music offered in the app is licensed and can be added. Creators who are performing covers, I believe, generally have the license held by TikTok or have their videos muted/taken offline. Special arrangements are made for intentional or encouraged content . That is a guess, but things like Megan Trainor’s “Gucci” where she is both the original artist and a participant would be a case like this. I would think Grace Kelly and sing alongs on arrangement-bound copyright material like Pentatonix doing public domain carols (or even Roger’s and Hammerstein) are negotiated licensing if outside of their pre-negotiated license.

No, they just put it as background music in their videos, but didn't actually pay for it. I would guess it constitutes fair use?