I mean, under the hood, it's just windows that are stacked on top of each other at different Z-index.
The wallpaper itself is a window, it sits at the lowest z-index. There's another (transparent) window a few levels above, where your icons sit. Dock/menubar sit on top of those again a few levels above. You can yourself put a window there at pretty much any level.
The screen saver is its own window that's put "on top" of everything else. The XPC I mention starts the player in the desktop window below. Makes sense ?
> I did not realize until you mentioned it that you could have different screensavers on different screens. I guess logically it makes sense given that you can have different wallpaper, but I am kinda surprised they took it that far.
They had too with the way they implemented it, for each screen if you want the "continuity" effect you have to be able to set a different one as both saver/wallpaper.
> I thought all of the videos are available on Mac. I can't think of any that I have seen on my Apple TV that I am not currently seeing as part of this. I am looking in system preferences and I see 61 landscape, 30 cityscape, 21 underwater, and 22 earth
Are you running Sequoia ? I double checked and it now looks like they use the full collection indeed. I'm fairly sure it wasn't the case in Sonoma at the beginning. Maybe they added them in a point update at some point and I missed them. They always had the full manifest of videos, but only a subset was available when they launched the feature, that I'm sure. Some are fairly old and not very sharp, so it made sense to be selective, but Apple does create new versions of the same videos to clean them up fairly frequently.
Behind the scene there's a SQLite database that used to contain only which ones were shown in the UI, while the manifest of videos (which is similar to, but not the one used on tvOS, which has HDR versions encoded with Dolby Vision) is pulled online periodically.