Google's Material Design (wikipedia) is much more widely-adopted across OSes/Flutter/the web (see how many websites have that dropshadow topbar and ≡?); Microsoft's Fluent (wikipedia) is Windows-first, but is usable anywhere.
Both are based on responding to user actions. Fluent uses lightup acrylic (translucent) canvases (e.g. hover? border glowy.)
while Google's Material uses paper-esque whitespace, navbars, dropshadows, and round corners. (e.g. scrolling? dropshadow appears on nav)
Think Microsoft Teams vs. Google Drive.
They're both full-fledged but Material You is way more common judging by places such as the F-Droid ecosystem on Android. As for which is "better", Material You supposedly has better colorscheme flexibility since it 'wants' to adapt to e.g. user wallpapers. But other than that it's really just preference (or whether relevant tooling exists :P). I know some devs use Material You for a predictable, unified look across Android apps, while others bend them to their will to reduce animations or whatnot.
If you're designing something, make sure you keep your own self in the mix too. Breezy Weather uses Material Design, but it's more customized to have a unique feel than, say, TrackerControl (which also uses Material).
Material Design is nice, but "Material You" is a drastic misstep and garishly awful, extremely unpleasant to look at and interact with all around with the gross pastels (especially that skin colour one ewww).
Hopefully we go back to the Android 8.1 - 10 design language as that was the best of Material.
Based on the other posts, I think it would fit right in over there. It only "fits" here because this community has no theme. It copied its name from the subreddit but didn't try to mimic what made it unique.
For reference, the subreddit was for "common sense" questions people might feel stupid asking. An example of a great post there was "how often do I have to wash a hoodie" or "do I really have to change my oil every 3000 miles?"