[zozbot123]

This is actually not the case. The owner is always made aware of whether the system has been opened up to "untrusted" software or not (they get a warning at boot if it has been), but an "untrusted" system can still be OTA-updated if the original OS image has been preserved as-is. This is often combined with some amount of custom modification by separately installing an "overlay"-based solution such as Magisk. But a full-custom ROM cannot be updated in this way, because the manufacturer's OTA update is monolithic and effectively replaces the original stock ROM!

[Arnt]

AIUI, an OTA update (or USB update) is effectively carried out by software that was booted from the same flash memory to which the untrusted software was written.

I accept that the manufacturer's OTA update is intended to be monolothic, is desigend to be monolithic, but what assurance do I (the owner) have that the software that was flashed by a physical user actually flashes its replacement monolithically? That it leaves nothing behind?

EDIT: on further reflection, it seems possible to design a phone that provides such an assurance. That any monolothic OTA update actually has to be monolithic, even if untrusted software is in control of the main CPU. But I wouldn't want to bet that any/many/most phones built today actually offer that guarantee.