How does YouTube verify the content license?

Question

YouTube

1) Let my video consist of free video (free use, royalty free) and free music. It is unique - this is not a re-upload of someone else's video. But, in other videos there may be the same video or the same music (in a different sequence, for example, with a different video sequence, etc.).
We all download from the same sites and then edit the video.
Will there be copyright issues on YouTube?

2) On different sites there is a large amount of paid content - music, video. That is, you subscribe to the site and buy video / music. Let me clarify once again - this content is for mass sale, posted on "stock" sites.
If the video is edited from such paid fragments, how will YouTube check the license for the content?
Indeed, as in the first case, this content can be used by many video authors.

3) And if the subscription is not purchased? That is, a subscription was "bought by a friend" and gave the sound file to you.
From a legal point of view, this is clear, but from a practical point of view?

How does YouTube verify the content license?