Installing rsync on Android to transfer files from Android to OSX
First on Android each app has an own user-id/account, hence u0_a312is the user of the app that runs the ssh server. Regular Linux user accounts like myuser don't exist on Android.
Based on the path /storage/6632-6330/ I would assume that you are trying to access the external sd-card.
On a non-rooted device write access to ext-sd is not possible on Android using standard Linux/Unix file programs (this is only possible via Android Java API). Only if the ssh/ftp server + rsync was specially adopted to use this API it can work.
On Android 10/11 the situation is even getting worse because of the new &quot;Scoped storage&quot; system. Not sure if it is still possible to get full access to all files on the external ad-card.