Location-based tracking. In this case, your phone was on your friend's wifi so they know whatever that wifi was doing while you were there, you were in the room

Location-based tracking. In this case, your phone was on your friend's wifi so they know whatever that wifi was doing while you were there, you were in the room. Unless you've already completely de-googled your life, this is the most straightforward way to link you to the activity. All it would take is for you to have pinged Google's servers with that phone just once, ever, and they've got it fingerprinted. So when it shows up on the same network accessing YouTube, they placed you. One use of Google Maps, Google Home, logging into Gmail or YouTube is all it takes to compromise a phone.