I would add one more thing on this.
Also those guys boosting videos through the playlists, that's what they're doing...
In that way they're boosting "channel watch time" and "session watch time overall", these 2 things are most important things in terms of ranking
Session watch time means how much time viewer stay on YT and how long watch the videos -> No1 factor for ranking nowdays
Let me explain you, if someone watch your video even 100% of it, and after that leave the YT, that wouldn't be that helpful in terms of ranking tho
But if that viewer after watching your video watch some other videos that would be a great sign to algorhitm that your content is a good one.
So that's why their bot watch the videos from different channels grouped in one playlist, ranking algorhitm getting triggered cos 2 things are fulfilled (Watch & Session time) and automaticly those videos clasify as "valuable content" and keep suggesting it between those channels.