As I’ve wrote above: I just use the python script for extracting the payload.bin:
Place the files extract_android_ota_payload.py, update_metadata_pb2.py and the [name of your rom].zip in a folder of your choice. Open a terminal and run the script with
python3 extract_android_ota_payload.py [name of your rom].zip
After that you’ll find the extracted image files in the folder you’ve run the script in.
I did that on a linux pc and didn’t tried that with windows yet. Maybe with windows this could be differerent.
About root and OTA updates:
Wait until the update is finished but don’t reboot your device yet. Open Magisk app, tap on Install and choose the method “Install to inactive slot (after OTA)”. After Magisk installation is complete you can reboot your device.
I’m doing this on a FP4 device since iode 2.4 with every beta update and never had lost root after it. You just have to take care installing Magisk after OTA update and before reboot.
Edit: after you have your extracted boot.img I suggest to follow the official Magisk Installation guide with flashing the magisk patched boot.img with fastboot. So you don’t have to install TWRP.