what is the best case for a macbook air 2021 13"

Apple recommends not putting anything on the screen. The space between the screen and the keyboard very small. If you make it even smaller with some sort of screen protector, you run the risk of cracking the screen. As the screen is not a touch screen, it's not going to get especially dirty.

As for a case, get what best meets your needs. I use a padded neoprene case with a zipper. It's pretty generic. It's small enough that I can still put the whole thing into a backpack or briefcase. I probably paid less than $15 dollars for it on Amazon and have had it for years (and a couple of different computers). If I didn't usually carry some other sort of bag that I put it in, I'd probably opt for something with a carrying strap.

Best of luck and enjoy your new computer!