I dunno.

Storing your files on a synced folder (i.e. DropBox or something similar) can handle a lot of heavy lifting.

The use case is not simultaneous editing, folks typically aren’t working on things on different systems at the same time.

Given that as a basis, next, “save early, save often”. The app is “cloud ignorant”. It just sees a local folder. Most of the syncing systems sync up quite quickly, so even if you save it on your desktop and pick up your phone, it’s probably already synced up. “For free”.

Finally your app needs to be cognizant that the file can change behind its back. When it fires up it reloads, or prompts the user, or whatever.

Adding in auto save and versioning like Apple does with their apps, and magic happens (though I honestly don’t know how the Apple apps respond if the saved document is changed underneath it).

There’s a difference between simultaneous and background changes. No reason to over complicate things for the vast majority of use cases.