There are several sub tracts in all the videos and audios in a page. Other than the added sub there are 2 kinds of subs in a video/audio:
1. Speech recognition subs
2. OCR subs
See more click here:
Speech recognition sub extension; OCR sub extension on Edge; Dictate sub - Microsoft Tech Community
Bing could search the CC subs in text sub to improve your searching. That is Tik Tok already do!