It sounds like subtle differences in Youtube's front end here from there. If I go in from a link here or from Firefox, no sign-in option is offered. Even before I had any account there never was a sign-in function. They gathered history and profiled in the same manner as a search engine.

Here, they started to nest commercials about the same time as Amazon Prime did, gold to be dug and all that. At that point, I did get an account so as to pay the tithe and be spared the nested commercials; before that I had no account or sign-in either. They still knew who (probably by IP address) and logged viewing history.

All that data mining nonsense is legal in the US. I suspect UK and EU privacy law restricts a portion or even all of what goes on here.