[OnePlus 7 Pro Guides, News, & Discussion] redpoint73:
Honestly, I've never used TWRP to install the OTA, so I can't speak to how TWRP handles A/B partitions. What you've assumed above about flashing, active slots, etc. is the same as I would assume. But again, I don't have a good understanding of how TWRP handles it. You can head over to the TWRP thread in Development section, and ask (the TWRP developer for this device) mauronofrio, as he will probably answer it (he is good at answering such questions).

For this device, I've always updated from directly from the phone settings (not from TWRP). There are a number of variations on this. But the simplest situation is when the phone prompts me there is an OTA update (and assuming you are updating and already rooted with Magisk):

1) Update as prompted by the OS (do NOT reboot)

2) Open Magisk Manager

3) Install TWRP installer zip as if it were a module (3-line menu, select Modules, and tap the "+" button, select the TWRP zip)

4) Still in Magisk Manager, on the main screen tap "Install" for Magisk. In the popup, tap "Install", then where it says "Select Method" pick "Install to Inactive Slow (After OTA)"

5) Reboot, and you should be updated with root.

This is pretty straight forward from A/B perspective as you and I understand it. You have updated to the inactive slot as expected. Installed TWRP (which actually installs to both slots), then flashed Magisk to root the inactive slot (which has just been OTA'd). Reboot the phone, which will put you on the previously inactive slot, which is updated and also now rooted. All as expected.

Another variation on this, for instance if you want to download the OTA manually and put it on the phone:

1) Have the OTA zip file copied to the "main" directory of the phone's storage (not in a folder)

2) Open phone Settings>System>System Updates

3) Tap the "gear" symbol. Pick "Local Upgrade"

4) Select the desired OTA file, which will install it (do NOT reboot).

5) Open Magisk Manager

6) Install TWRP installer zip as if it were a module (3-line menu, select Modules, and tap the "+" button, select the TWRP zip)

7) Still in Magisk Manager, on the main screen tap "Install" for Magisk. In the popup, tap "Install", then where it says "Select Method" pick "Direct Install".

8) Reboot, and you should be updated with root.

Now this is different from the previous method, as it installs to the current slot, and then re-roots that same slot. Reboot, and it should be on the same slot, updated and rooted. Again, there are other possible variations on this. You can probably update by putting the OTA file on the phone, and installing to the inactive slot (maybe copy to .Ota folder?) and rooting the inactive slot. But I haven't tried it that way.

The second method is pretty much what is described here: https://xdaforums.com/oneplus-7-pro/how-to/successfully-upgraded-to-9-5-7-t3937414

Which I've used to update many times, without a hitch. It has been updated with the "TWRP retention script" which I haven't used. It used to include an additional step of flashing Magisk directly (in addition to the inactive slot), which I believe is also related to TWRP retention.

I've rambled on far too much. Not sure if this all helps or not. But I've used the above two methods (usually the 2nd) to update many time and never had a problem.