YouTube controls iTunes and vice versa

Hi, on a MacBook Pro with High Sierra connected to a bluetooth receiver (if any importance).

1. I listen to music in iTunes then I open an YouTube video in Safari.
2. Then I click on pause on the YouTube video in Safari (you know, on the graphical interface button, not on the physical keyboard) and it stops the video but also stops the iTunes music.
3. I wonder what happened, I switch to iTunes, I click on the play button (you know, on the graphical interface button, not on the physical keyboard) and the music starts again but also starts the YouTube video in Safari.
4. I switch back to Safari and lower the YouTube video sound volume. Luckily it doesn't lower the iTunes volume this time.
5. Then I click on the video progress bar to go forward the video. While mouse-down the iTunes sound is stopped, when mouse-up the iTunes sound is back (you know, the iTunes sound)
6. Then, to end it all, when the YouTube video ends (itself, not me stopping it) the iTunes also stops, for good.

Now, I've heard about this new super feature, that media buttons now control everything, not only iTunes or Spotify but I'm talking about the graphical user interface here, not the physical media buttons. I guess this is somehow connected.

And yes, I heard about an app that could possibly solve this, and another app that hides the menu bar icon for the first app but, you know, too many apps.