That "firmware" which is being replaced by open equivalent is a GNU/Linux system prepared by Quectel running on the modem itself and providing services like AT command or audio routing daemons. The actual baseband-y stuff is done in Qualcomm's DSPs which stay as closed as ever.
Pinephone modem runs its own OS inside. So if you have a Pinephone, you run two Linux'es at the same time: normal OS and the one on the modem. The core of modem responsible for the actual connections to the towers is closed though AFAIK.
Sorry, the phone world is still very foreign to me. I realize these questions may sound silly, but I'm slowly piecing all of this together.