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It's actually a limitation, Android does not allow apps to write files to anywhere.
But in your case, Google Photos should be possible.
Only when you want to write to the external SD card, and the developer can set a flag that it is allowed to do so.
On the internal storage an app can write anywhere on internal storage (/storage/emulated/0). From Android Q (10) on, there are certain dedicated folders in which you can write a file, but the user can still choose or select a folder inside that tree.
Ao, anyway, this can be a good feature.