I highly recommend installing an extension that hides the home feed and sidebar recommendations, which at least makes YT non-distracting again.
The phone camera sensors often have a aspect ration of 4:3 but the sides are cropped in software. So the videos just get mutilated because convention.
Though at least 4:3 format is making a come-back because it is the prefect comprise format. Looks great on a tablet, is usable in both landscape and portrait mode. On Desktop it leave space to read comments. Perfect for youtube videos.