I've noticed that watching literally part of one video will cause youtube to recommend a lot more of that category of content.

For one recent example, there was an article about Elon Musk and his girlfriend who releases music as Grimes. I clicked play on one of the embedded music videos and watched about half of it. Ever since then youtube has recommended tons of Grimes videos to me.

I've also noticed that videos my wife or kids watch get recommended to me sometimes.