The format and generation of the CYW43 firmware is quite complicated. The make_firmware project tries to compile the firmware from "source" binaries. PICO_SDK_PATH has to be defined so that the results can be copied to the pico-sdk/lib/cyw43-driver/firmware folder. See the makefile for more details. All firmware is stored in binary in header files with appropriate macros to define its size before and after compression.
By default the Wi-Fi and Bluetooth firmware is embedded in a PICO_W binary. The PICO_W program flash_firmare is defined as an example to show how you might flash the firmware to a devices internal flash. The first block written describes the firmware details (see cyw43_flash_firmware_details_t) and the firmware binaries follow.
The project test_flash_firmware is defined to show how to change how firmware is loaded. To configure a PICO_W program to retrieve the firmware from flash you need to change cyw43_get_firmware_funcs. CYW43_FIRMWARE_FLASH_PAGE should be set properly to be the flash page containing the firmware details. As the program no longer contains the Wi-Fi and Bluetooth firmware it should be around 150KB smaller (230KB if firmware is uncompressed).