There are designs where the modem (and other parts of the system which run closed firmware blobs) is a separate HW entity which is connected to the main processing engine via a serial bus. In these cases you can treat it just as you describe, a black box that transmits your data up and down. However, in some other designs the HW running closed blobs can directly access the flash&main memory of the device, and you have absolutely no guarantees on what it is reading/writing there (and when it does so) The Qualcomm chipsets fall in the latter category