This subject is compounded by the fact that there are both soft and hard backdoors in existence, example of soft backdoor, a recent report of a popular NAS supplier forcing an update onto their users devices, an example of a hard backdoor, Intel's management engine, a chip on the motherboard.
Both are non hidden backdoors for legitimate purposes, but are backdoors none the less.
Do you know each and every chip and it's function on the motherboard for a PC/ laptop, or each and every chip and it's function on a phone's circuit board?
I remember a story some years back about a chip found to be installed on popular motherboards by Chinese manufacturers rumoured to phone home when an internet connection was present.
Can't remember the exact details or what the outcome was, but it is entirely possible for any hardware manufacturer to install a chip for 'purposes' and to hide/ mask it's presence.
My grandfather told me to get the good things in life before it is too lateI would just get good AV and not worry