It would appear that Apple licensed a number of fonts for th MacOS Big Sur release which it decided not to use. They are however, still available to user if you want them. Some of them are really quite nice - you didn't really think Apple would buy ugly fonts did you?
So how can you get them? It's not hard, you just need to know where they can be found.
Go to Applications/Font Book (or select it your other favourite way, Launchpad, Spotlight etc)
Click on All Fonts in the left hand pane
Now simply look for any greyed out fonts (like Adelle Sans Devanagari shown below).
When you click on them they will display a sample and you can decide if you want to download it. If you do, simply right-click the font and select 'Download "font name family".
That's it, work through all the ones you want and you'll now be able to use them in any application which supports them.
Unfortunately there's no simple way to download them all at one time but it's probably better to work through and just download the ones you really like.