Thanks for the examples. I simply wasn't expecting the features you originally listed to require application support, I see they're more involved that I imagined.

You bet. Even with things like iCloud, any old app can store a file in iCloud Drive just like any other folder on the computer, but apps have to use the CloudKit SDK to use more advanced sync features. It keeps getting updated annually, so apps using an older CloudKit can't use features that've been added since then.