Consider Intel ME firmware is a literal well documented backdoor we do not allow on US government systems... only civilians.
Most of the time our adversaries do not need to be covert enough to mess with firmware. Consider OMA-DM apps that run on most phones with insane permissions taking orders from cell towers.
https://gist.github.com/thestinger/171b5ffdc54a50ee44497028a...
We cannot even keep public open source repos like NPM free of supply chain attacks. Proprietary blobs make it that much easier to hide things.
Also all you need to backdoor every encrypted messenger is a kernel module that ensures /dev/urandom is a bit less random on the devices of targeted dissidents and journalists. Now look at how many proprietary blobs from piles of random vendors we load into modern phone operating systems, even "open" android roms, and think about SolarWinds for a second.