How do YouTube downloaders access the video?

Question

I found this answer , but uses YouTube APIv2 and is now deprecated as indicated in the answer.

YouTube’s terms of service indicate that you cannot use another player or upload a video, so it makes sense to exclude it from the API.

Knowing that, like youtube online downloaders or third-party Android apps (which allow you to turn off the screen when they continue to play music, for example), access these videos?

I’m trying to access a video in order to transfer it in unity, for example (by the way, there are several unity assets there), and I would like to know how they do it.