Can I download OS but save for installation later?

Okay, great. Thanks.

I"m just trying to get myself organized with backups and etc so that's why I"m going to download Catalin. However, now I"m a bit confused because it's says on the app store beside Catalina that it's "compatibility 10.9 or later" . . .

I am on my 2009 MBP (el Capitan 10.11.6) right now and I thought it couldn't support a more recent OSX? Am I misunderstanding this?

Yes that why I've only even run that and never tried to upgrade to Sierra or beyond. Why does the app store have under Catalina information "Compatibility OS X 10.9 or later".

I"m still downloading for later as I have a new MM but when I read that bit it just got me curious.