I've looked around quite a bit but haven't been able to find much of anything in the way of documentation for this feature.
How does it work?
Does it load random videos by specific artists?
Similar type songs, like Pandora?
Does it only use content from specific users/channels?
Does it load a certain number of videos?
What happens when it gets to the end of the list, if there is an end?
Is the list (relatively) infinite?
Is there any documentation about it?
YouTube Mix is an auto-generated playlist that will display suggested videos based on what you’re watching. This is the best description that I've found, and it is consistent with what I have observed as a YouTube user and channel "owner":
YouTube Mix offers users the ability to watch 50 videos continuously that it thinks they’ll be interested in. No additional work needed to click through each video one by one to watch it. Each playlist features up to 50 videos, but YouTube says that once all of those videos have been watched, an additional 50 will be shown, showing users more content that they are interested in. Just like a music playlist, users can shuffle videos, place it on repeat, or skip around to specific videos they wish to see.
Mixes are created automatically and randomly, and calling the same playlist by a bookmark or a link never leads to the same result again—see Is there a way to save the YouTube MIX playlist in my playlists? Google doesn't disclose the selection algorithm used.
Despite what some say,the YouTube Mix feature is not gone. It is available on web and mobile.
