Why can't I watch videos on my phone and PC at the same time?

I have a pretty annoying problem. If I watch YouTube, Twitch on my PC and then I try to watch a YT video on my cell phone, the playback on my PC stops. The other way around I have the same problem. The two devices are not connected and are not even connected to the same Google Account.

Unfortunately, that doesn't make sense unless you use e.g. The same keyboard for both or something. You have to be connected in some way. Otherwise there's no such thing.

Check if you haven't connected anything to the other.

I found out. My cell phone was connected via a Bluetooth speaker and now the problem comes into play and why I didn't suspect it. My PC probably connected to the speaker automatically without using it. That didn't come to my mind either, because the sound came from the PC through my headset.