For example this clip on Vimeo https://vimeo.com/channels/staffpicks/361821058 in the bottom-right, one-to-last button is the popout button. The clip will now hover over all other windows. Other websites have other buttons, or have this in the generic right-click menu. And some simply don't offer this.
If you're asking for a way to keep any window on top in macOS, there isn't a way. The above also works in Safari, but not in Firefox.
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.