Hi @davepatton
If I understand correctly you’re trying to use/implement FOTA and you’re trying to figure out the inner workings. You can check out this sample which uses FOTA via HTTP
nrf/samples/cellular/http_update/application_update
More specifically it uses the CONFIG_FOTA_DOWNLOAD
CONFIG_DOWNLOAD_CLIENT
and CONFIG_DFU_TARGET
to write a .bin image into the free secondary slot. You’ll have to dig in like to nrf/subsys/net/lib/fota_download/src/fota_download.c
to get the answers you’re seeking.
Hopefully that should help!