Wayland isn't a window manager or a desktop environment, it's a display server protocol. I don't personally use Mint, but I skimmed through the video and it seems if you update to the latest version of Mint, you can use the Wayland session, but it's still very work in progress and you should probably just stick to the X session for the time being until they work everything out and get it to a more usable state.

And for the difference between a window manager and a desktop environment: A window manager is pretty self explanatory... it manages windows! It's what handles the positioning, sizing, etc. of windows (simple explanation). Examples of a window manager are things like i3wm, swaywm, bspwm, awesomewm, KWin (KDE Plasma's window manager), Mutter (GNOME's window manager), etc.

A desktop environment is essentially a "suite" of things (including a window manager!) that most people would expect from a computer, things like a status bar/task bar, control panels, etc. Examples of a desktop environment are things like KDE Plasma, GNOME, MATE, Xfce, Cinnamon, Budgie, etc.

I simplified things quite a bit, but that's the jist of it. If you want to get into semantics, technically KWin and Mutter are also Wayland compositors, but that's a whole other can of worms. [insert your favorite search engine's name] is your friend if you want to learn more from someone who can probably explain things way better than I can, but hopefully I was able to somewhat help clarify the differences, lol.

One way to tell the difference is to use a distro that doesn't have a desktop environment like Bunsenlabs. Bunsenlabs doesn't have a DE, but it uses a windows manager called Openbox. The menu and the theme of the windows is controlled by Openbox, the panel at the bottom is Tint2, the file manager is Thunar, the application that controls the desktop wallpaper is Nitrogen, and the display at the top right is Conky. All of these applications are independent applications with independent developers, meaning they aren't really designed with each other in mind. Nothing's stopping you from replacing Openbox with a tiling manager like i3 but keeping everything else the same. It's for this reason that Bunsenlabs doesn't have an actual DE.

In sharp contrast, for something like Gnome or KDE Plasma, the suite of applications that form the DE are very much designed to work with each other. You don't have to stick with the default applications and some DE's like xfce are designed to be modular like that, but it's a pretty janky experience. For a while, I had an xfce DE except that I used Cinnamon's file manager Nemo instead. But since Nemo was designed to work in the context of the Cinnamon DE, there were certain features that Nemo should be capable of but I was unable to use because I was using xfce instead of Cinnamon.

If you upgrade to 21.3, yes you can use the experimental wayland session. However, Mint will still default to X11 until around 2025-26 when wayland support becomes finalized.

On an aside, I don't know if downstream distros (distros other than Mint) will package the wayland session alongside X11 as well, but Mint definitely does.