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Restoring encrypted files from backup to an iPhone is a similar feature request, and explains issues.
Basically, the backup format is so very different from the source that lots of conversions are needed.
How the backup process works
How the restore process works
Because there’s no server-side processing to help (it’s just storage.here, unlike with some systems) downloading and processing can add delays, similar to Direct restore from backup files (try that out).
Maybe at some future time when/if Duplicati migrates to something Android can run, it will be able to Direct restore there, but it might still be a slower experience than if a backup server can help out.
Introducing .NET 5 sounds like that may increase portability of programs between operating systems.
Net standard migration #3124 is a very early step in that direction, but migration isn’t going to be fast.
Duplicati on a phone would probably be a subset (easier?) but might want a non-browser UI (harder).