Music

Do you mean why some people are able to sing? Or why they're able to use popular music in their videos?
If it's the latter, that has to do with the songs being licensed for use with TikTok. If the distributor for the particular song has released it on TikTok, then users are allowed to use them in their videos. If they have not, you'll probably get a copyright strike/video takedown on your video.
An alternative is that the person who made the video has bought a 'mechanical license' that allows them to make a cover of the song.

Yeah this question seems a bit vague- but short answer some have talent 😂😂