[Google Pixel 6a] V0latyle:
Well, I had thought there were existing guides in this section, but either I was mistaken or they've been buried.

Maybe explaining it could help you. I'll do so in response to your questions below.

Exactly.

Regardless of whether installed through system update or via sideload, the OTA package always installs to the other slot - if you're currently running on slot A, it installs to slot B, and vice versa. See A/B (Seamless) System Updates

OTA through system update doesn't always seem to work for rooted users; some have suggested that it may be necessary to restore images in Magisk to pass verification checks in order for the update to succeed. Others, like me, don't bother with the OTA updates anymore because 1) OTA through system updates just don't seem to work and 2) Magisk patching inactive slot still seems to have issues. I personally prefer to use the factory images, but the OTA method is technically safer.

OTA sideload should always work, and AFAIK it doesn't matter what state your device is in. It overwrites whatever is on the currently inactive slot; there's no need to remove modules or restore images or unroot as the boot image gets overwritten with the new boot image.

No, they aren't.

When your device boots, it loads the contents of the /boot partition into memory - ramdisk and kernel. This remains in memory while the system is running.

Hence, if the boot image has been patched, you'll boot with root.

When you perform "Restore Images" in Magisk, this uses root access to restore the unpatched image to /boot. But, since the patched boot image was already loaded into RAM, you still have root.

If you reboot at this point, you'll lose root because the now unpatched boot image is loaded into memory.

Yes and no.

After an OTA update, you want to let the system boot with the new boot image. This is because the boot image has a signature that matches the system image; if these don't match, the update engine determines that the update was a failure, and will recycle you back to the original slot. So, the boot image that the system loads after the update has to be the new one. It doesn't matter if it's been patched or not - you could, for example, download the factory image, extract the boot image from it, patch the boot image in Magisk, then sideload the OTA, force your phone to boot into bootloader, and flash the new patched image.

Patching to inactive slot in Magisk is supposed to avoid all this, and can only be used if you update through system. It works like this:

You're rooted on the February release on slot A.

You restore images in Magisk, but do not reboot.

You install the OTA through system updates. This installs to slot B

When it prompts you to reboot to finish the update, instead you go back to Magisk and use Patch Inactive Slot - this uses root access to perform the Magisk patch on the new boot image in /boot_b

You then reboot, and the system switches to slot B, and loads with root because you patched the boot image.

However, this doesn't seem to be working for everyone. Some phones don't even "see" the OTA update is available for some reason.

Therefore, this is the method I recommend you use to install the OTA update:

Download the OTA update for your device from Pixel OTA Images

In Magisk, go to Reboot > Reboot to recovery

Once in recovery mode, hold Power and click Volume Up. Then, select "Apply update from ADB" and connect your phone to your computer.

On your PC, open a command line in your ADB Platform Tools folder, and type adb sideload <path to OTA zip> - to make this easier, just drag and drop the OTA zip into the command window.

Once the update completes, let your phone boot to system. You will not have root, so it won't matter if you didn't disable Magisk modules.

Let the system complete the post-update work (there's a notification with a gear icon) and get stabilized. Wait a few minutes.

Assuming you still have USB debugging enabled, use ADB to reboot to bootloader: adb reboot bootloader or you can just do it manually - reboot your device and hold Volume Down

You can now live boot an older patched boot image: fastboot boot <drag and drop patched boot image>

You should boot with root. Note: if your phone bootloops, see my note below)

In Magisk, go to Install > Direct Install. This uses the "temporary root" of the patched image you've loaded into memory to patch the new boot image

Reboot once more. You should now be on the new boot image with root.

If your phone boot loops, double check to make sure you're still on the latest update. You can check this in Settings > About, but it's also displayed on the notification windowshade under the Quick Settings tiles. Make sure this matches the OTA image you just installed. If it doesn't, this means that your device has recycled back to the "old" slot for some reason, and you'll have to start over.

If you want to play it safe, you can use this guide to extract the new boot.img from the OTA payload. You'd then patch this manually in Magisk, and after installing the update, you'd reboot to bootloader and flash to /boot.

If you still bootloop on the new boot image after patching, you probably need to disable Magisk modules. You can do this via command line:

adb --wait-for-device shell magisk -remove-images