Downgrading is not as simple as pressing a "back" button. It requires unlocking the bootloader and using developer tools. Below is a generalized outline of the procedure.
On Android 12, the standard "Local Install" option is often hidden. You may need to download and install the official OnePlus Local Update APK Prepare the Rollback File: Downgrade Oneplus 9 To Android 11
How to Downgrade Your OnePlus 9 to Android 11 (OxygenOS 11) If you've updated your OnePlus 9 to Android 12 or 13 and found it buggy or simply prefer the classic OxygenOS experience, you can roll back to Android 11. This process is official but , so a full backup is essential before starting. Prerequisites Downgrading is not as simple as pressing a "back" button
He didn't restore from backup. He installed his apps manually, one by one. By midnight, the phone was his again. On Android 12, the standard "Local Install" option