I actually don’t know how they do that. It’s quite scary.
That’s the whole point. 
The most eerie one I can remember recently is that once I was watching something on youtube in a room, and the stuff I was watching showed up on the person in the next room’s youtube feed. That was insane.
They were using your premium account without you knowing.
That’s not insane, you were probably just using the same IP address and YouTube thought you were the same person
Not really. Depending on what the video was, it could be something really normal. Maybe just a popular or trendy video? Also coincidences exist.
But apart from that, and more related to the FB and google ads… two things:
- The profile people and make segmentation, and because you mach with certain profile, they imply that you’re likely to be interested in X, Y and Z. Also they probably cross reference connections and interests
- But I also did a test one day, and it was scary shit. I didn’t have a dog or a cat, told my friend we were going to do this test during lunch time, I opened the phone, left it on the table and start to talk about pet, pet food, I need to buy food for my pet, etc… or maybe it was toys, can’t remember. I’m sure I didn’t have any pet food or toys search in my google history. What ad did I see next morning? Yes.
Not even that, they can use IP for determining what content is relevant based on previous reproductions even for different accounts. But I don’t work for Google, so I can’t be sure 
