This is anecdotal, for now, but I can personally confirm that all your online accounts are linked to one another and whatever network that oversees them makes calculated moves depending on what you do on them.

Let me explain.

I've had a YouTube account for a while, we'll call this accountA. When I started work about 4-5 years ago I made a separate one, accountB, on a different e-mail address so they were completely unconnected purely for work purposes, like listening to music/podcasts and looking up photoshop and programming tutorials for work.

After a while, they started to blend together with similar recommended videos, but it doesn't stop there.

Say I watch something on accountA at home. When I get into work the next morning I'm surprised to see nearly all the same videos I watched the night before in my recommended video list on accountB. This isn't a one-time thing. This is pretty consistent.

Likewise, the same happens in the reverse. I'd tested it by taking up a hobby of tying knots. I'd set up a couple knot videos at work and listen to them while working. After a while, the same merging scenario started to happen. Now both accounts are recommending knot videos, but like I said before, they are now recommending the exact same videos that the previous account had already watched, with little to no delineation.

There are other instances of recommended videos that follow suit. At this point the accounts' recommended pages are almost 1:1, despite me typically viewing completely different content on them throughout the day. Somehow, YouTube/Google knows I'm the same person operating both accounts.

I've only just begun to catalogue the nature of these accounts, so bear with me on the lack of specifics. It is more than just a hunch or gut feeling. I'm pretty sure at this point that social media sites can easily pair accounts to one another and act upon that data.

I'm not sure it is worth following up on, but just so you know...