iCloud already supports this, and from what I gather it works decently well:

https://developer.apple.com/documentation/uikit/uidocumentst...

https://developer.apple.com/library/archive/technotes/tn2336...

Essentially there's an OS mechanism for getting the different versions of the file, your app can detect conflicts and choose how to resolve them, including by displaying arbitrary UI to the user. IIRC This UI can even be integrated with the file open dialog. As a last resort fallback if the app doesn't resolve it Finder/Files will let you keep either version or both (as separate files)