Can u I use an upgraded iPhone to complete recovery mode
I want to restore the iPhone 6s using the iPhone XS max
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.