Vendors with nefarious intentions or operating on directives from a state agency are a real and valid concern, but the handset under consideration makes use of a Qualcomm baseband chipset, so the OS it runs on is supplied by Qualcomm, not the manufacturer.
If I understand it correctly (but please do correct me if I am wrong), Qualcomm supplies the baseband OS as a signed binary blob.
My knowledge is third-hand as I haven't worked at Motorola for over 20 years (and back then I worked on "server" side, not device side), but I think you are correct. What I'm unclear on is how it differs from the chips (and associated blobs) sold for use in China, where the legal requirements differ, and how reliable the "signing" really is. I do know that some groups get special permission from Qualcomm to load other firmware and expose certain features to the user-land OS. My threat analysis is mostly theoretical and I can't really say that Qualcomms control is lacking or co-opted.