Yt am pc poor quality?

I actually have a very good pc on which I can play almost any game on high settings fluently and have great graphics. My problem is only that if I yt s.pc on the browser watch the quality is mieserabel although I put on 1080p. On Twitch etc I do not have the problem

Could be because your Internet does not do this.

But on the phone, etc. I also have perfect quality

Can your screen be 1080p? Resolution set correctly? Possibly. Update the browser.