To clarify, 4G modem modules (like the one in PinePhone) are usually based on stripped down smartphone SoCs, running Linux on ARM + a huge multi-milion lines FW blobs running on some DSP, that's actually implementing most of the functionality you expect from the modem.

So what this means is that PinePhone is basically a smartphone withing smartphone, and what's opensourced is the Linux kernel running on the modem's SoC. It's very nice, and allows you some more control (now you get two Linux capable SoCs in one phone that you can fully use for whatever you want, which is quite interesting!), but the actual modem part is as blobby and impenetrable as ever.