Perhaps less relevant to the likes of Netlfix, but youtube are certainly tracking much more than just what you watch on youtube even if you pay for no ads. They will be recording details of all the sites you visit that embed their videos, and so forth.

And while they aren't going to be sharing exact data in any way, that doesn't make business sense, I'm sure there is some sharing of indirect tracking data (cohort flags & such), as I've seen activity on youtube apparently affect what I get hawked in recommended links elsewhere.