Can u I use an upgraded iPhone to complete recovery mode
I want to restore the iPhone 6s using the iPhone XS max
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Moving from a lower to a higher version was never an issue. You can't restore a backup from higher iOS to a lower one.
Please see the pic below which is self-explanatory. Here you have a backup taken on iOS 15.5 and trying to restore it to iOS 12.5.6 which is not feasible.