You can download the schematic of Cellular IoT HAT from this github repository.
You can download the 3D design files of Cellular IoT HAT from this github repository.
The Raspberry Pi Cellular IoT HAT github repository contains the libraries and codes are used in tutorials, hardware files and more.
The Firmware for BG96 is given below. Please check the module version on the HAT download the suitable version.
Click to download the QFlash tool package