What an unfortunate lack of functionality. On XFCE, a free Linux window manager, I can right-click a window and select `always on top`. MacOS is a premium product and doesn't offer that.
One redeeming grace is that macOS does support virtual desktops quite well. That, combined with a nice 32" monitor, means I no longer need it as often. But it's a miss, to be sure.
Edit: if the "popout video" button isn't there for a video on a website, then it's in the right-click menu. Of course, some (most?) websites including Youtube have a custom right-click menu. To get it back, there is an extension, but boy what a hassle.