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...
I have another example. You know how if you mention a product out loud that your FB feed will suddenly start showing ads for it? I was recently working in a setting where most of us didn't keep our phones on our persons. Once in a great while we'd show each other a pic on our phones in passing in the break room. So when I started seeing ads for a craft another person at work is doing on my FB I started trying to figure out where they were coming from. At first I thought it might be because my daughter and I had discussed it. But then an ad for the company that this person's husband works for appeared on my FB. I had no other association and had never uttered the name myself. I figure it's happening via the free wifi at work. All those stores offering free wifi so you'll use their app while you are shopping? Prob using the apps to spy. It might even be happening while our phones are in our lockers in close proximity. Think about it.
I have two phones now. One I leave at home because it dies if unplugged. Another phone with another number that I carry for texts and calls. Zero apps on it, leave blue tooth OFF and WiFi OFF most of the time. Keep cameras covered. Not even email on the 2nd phone. Both with cheap service from different carriers.