I have a OnePlus 6 that uses the A/B partition schema.
Previously with my OnePlus 5 with TWRP, I normally download the OTA package, reboot into TWRP to install it, and then TWRP img and Magisk ZIP afterwards. Then I have a fully updated system with TWRP and Magisk.
Now with A/B partition, the OTA gets installed immediately after downloading. I can only flash Magisk before rebooting, by manually extracting the boot image using command-line tools and Magisk's "Patch Boot Image" function, before flashing the patched boot image back. However, I see no option to install TWRP in this way (I'm currently not able to apply the ZIP using CLI tools). Any ideas?
Some good points for OnePlus over AOSP: OnePlus detects Magisk root and downloads full image instead of incremental OTA image, which saves the time for boot restoration, and also allows installing OTA even if /system is modified.
However, there's no "Disable Auto OTA Installation" in Developer Settings, so OTA installation may not be avoidable to some extents.