The cellular modems are closed source. There are no open source cellular modems. And if you built one it would be illegal to connect to a carrier in any Western country. Your cellular modem has your IMEI, your SIM number, and unless you unplug wires, your GPS coordinates at all times. It communicates with your manufacturer and your carrier independently of your phones' OS. It has access to all outbound unencrypted data (and all encrypted data's metadata) whenever you're not on wifi. The rest of a smartphone is no more evil than a "cheap" laptop. Which is pretty evil and stalker-y, ngl, but cannot hold a candle to the cellular modem. I use GrapheneOS, but a "dumbphone" is at least as private.