especially since its radio and thats not too easy to determine if a device is or isnt sending weird stuff. (dont come with the lte or wifi sniffers or such things. u'd need thorough spectrum analysis during operations on a quite broad spectrum too to rule that out. the antennae in the devices can produce a lot of types of signals... or do they decap the chips and reverse those to see whats in it? i doubt it would be possible at the right scale but theres options i guess.
people in certain regions/ high assurance security work roles will do this to hundreds of devices that are identical to try and determine if a supplier is compromised or not. order a full batch, take em all apart. taking x-rays, dissolving chips package, etching layers one by one, taking pictures with electron microscopes etc, probing bond wires in the packages as they run etc etc.
somehow i dont see some OS creator do all of this, but ofc i could be very wrong :). interested to find out why people think google while with a different OS is truly de-googled or if they kinda just hope for the best...