For the uninitiated:
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Every Intel platform computer built since about 2007 contains something called the “Intel Management Engine”. It’s a subsystem that works invisibly at a level below the application processor, and has complete access to memory, as well as its own connection to the internet. The NSA was caught buying Dells that had these spying features disabled, something they called the “high assurance platform”.
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AMD processors since 2013 or so have had something called the “Platform Security Processor”, a similar unaccountable subsystem, although this one hasn’t been proven to be malicious, as far as I know.
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Most ARM chips used in cell phones have the modem baked-in on the same silicon die as the application processor. The modem’s firmware is a complete black box, and the modem usually has access to main memory.