In incognito, I get nothing similar to what I would actually watch. Nothing is even remotely similar. It's scarily different, and none of it interests me.
Same here. 100% generic clickbait crap and exactly the same stuff that shows in a Ungoogled Chromium incognito window. I hardly ever use Chromium.
I use Cookie AutoDelete, uBlock Origin, Decentraleyes and a VPN for casual surfing though.
When they see a new profile they are basically testing the waters. They suspect it's you, but leave the benefit of the doubt in case it's a legitimately new user. But just giving them a few "fuzzy" data points like watching a video or two will give them enough confidence to fully link your previous activity to the new session despite never providing any concrete evidence such as logging in with the same account.
2) This is what I would do if I had no morals, and the people working for Google are smarter and are paid way more money than me, and Google itself has an incentive to stalk people across different sessions, so if I can think of this and implement it then so can they and you'd be foolish to believe they wouldn't do it.
You'd think this is conspiracy-level thinking (and I would've agreed with you 10 years ago), but so far, when it comes to online tracking, everything that's been considered a far-fetched conspiracy in the past ended up being true, so better safe than sorry? Given the money at stake in adtech and lack of morals and respect for the law (Google still ignores the GDPR), if it's technically possible, you should assume someone's doing it.