Is either one of these hypotheses correct?
I haven't experimented with A/B device so far but what I understand from official documentation and source code is that it's both. SHA-256 hash of target partition is checked against the one received from update server. And after reboot dm-verity check is performed.
From Life of an A/B update:
"5. The whole ...
You can only install updates that are explicitly created to run on your device. Those update files are usually automatically downloaded by your device over Wifi and then automatically installed using the mechanism you already discovered.
Usually for cheap device like yours the manufacturer never releases any major updates (updates that upgrade the device to ...
Turn off your Data and turn on "Flight Mode", now if Flight Mode gets reset you still have Data turned off (and Update won't reset that, because there would be a flood of complaints).
I have always done my updates over Wi-Fi using the settings to select that option. Using the Towers to Update would be expensive (for me).
I updated Android recently. No Data ...
The /oem partition is used for any carrier customisations to the Android firmware; if your firmware build is not a carrier-specific one then it might be empty anyway. This purpose is described in the Android Project Treble documentation.