Root LineageOS 19.1 ( Android 12) using Magisk and install OTA update - OnePlus 6t (fajita) A/B Only
How to install root (Magisk) on Lineage 19.1 (Android 12) - OnePlus 6t (fajita)
Then install OTA update. Root is preserved.
[EDIT]: It seems that this method only works with A/B Devices that support seamless update. You can use Treble Check app to see if it will work for your device.
Magisk Download: https://github.com/topjohnwu/Magisk
Rename Magisk apk to a .zip
Sideload the file in recovery
Boot into lineage (connect to wifi)
Open the magisk app and allow to download and install
0:00 Intro
0:10 Download Magisk
0:33 Rename the apk to zip
0:50 Reboot device to recovery
1:30 Apply update - sideload Magisk
2:25 Boot into LineageOS
2:55 Magisk app installation in app drawer
3:47 OTA Update
5:14 Updated Lineage with root preserved