MediaTek is a Taiwanese company. Xiaomi could put any backdoor that the government tell them to install and TSMC won't be able to catch that.
There are accounts of interdiction and post-manufacturing compromises, such as those revealed in the Snowden leaks (2013) and most recently by reporting about the Israeli security services' sabotage of Hezbollah pagers and 2-way radios. These projects didn't expose primary manufacturers to the reputational and legal risk of association.
Until there's evidence of backdoors implanted by these companies, it's better to adopt a "trust but verify" approach and use standard layered security practices to detect intrusion. Conventional network intrusion and insider threat campaigns carry less risk of failure and are simpler and cheaper to execute.
Now, if the EU would start make to make chips that have no subsystem to screw me over, that would be something.
Why would they not do the same? They are very pro-surveillance for a start.
They are backdoors, with not even very good plausible deniability.
It's so on the nose it's comical.
Would Xiaomi or the CCP accept the reputational and legal risk of getting caught with a backdoor? Recall the CCP considered the arrest of the Huawei executive in Canada as an offense against itself.
The answer to that question in the US is within epsilon of “no” thanks to the cloud act.
Europe and China have passed similar laws. However, China has a reputation for passing laws then selectively enforcing them. Maybe if you get on the right side of the right party member, the answer might still be “Yes” there.
Anyway, the next question to ask is: Who would I rather backdoor my device?
I doubt the Chinese government cares much about me, and unless I vacation there it’s an international incident if they, say, hack my car and murder me by aiming it at a tree at 95mph, like the CIA apparently does now:
https://www.motor1.com/news/138679/wikileaks-claim-cia-hack-...
Of course, the US government banned cars from China. Presumably this is because they do not have the mandated remote murder network port open, or, if they do, the US doesn’t have the necessary permissions to access it.
For instance election tampering in the US is a well-documented thing: However it almost always favors republicans in districts with electronic voting machines that do not generate paper trails.
As for the person you are replying to, their claims have been documented by most media outlets, places like wikileaks, and their claims are backed by US legal requirements to backdoor stuff (like the US Cloud act).
A bit of selective memory necessary for that take given good old Crypto AG