This is no more required for FP3, and will be removed from the flashing instructions. We now embed the latest FP3 firmware (speaking here of firmware, not full stock ROM as in flashing instructions) in iodéOS. Generally speaking, the firmware is a set of binaries that perform low-level control on the hardware and various tasks to boot the device.
Although we do not support other recoveries than the ones we provide, they generally work. There are exceptions, for example recently to switch from iodéOS v2 to v3 on some devices (Samsung series 9 and Xiaomi Mi 9), due to encryption problems when formatting: our recovery is mandatory.
Fairphone permits to relock the bootloader of their phones with custom roms. That’s fair from them On most devices it is not possible, as the encryption key used to verify the boot process is not public, so cannot be used to sign the kernel, and cannot be changed. Fairphone uses the google public key given in AOSP, and in addition permits to provide a different key. For FP3 we did not change that key, but for FP4 we provide a custom key to enforce security.
Flashing without mentioning the partitions a or b happens on the active partitions, which will then be used to boot. In that perspective there are some flashing instructions for other devices that could be simplified.
We always ship full roms as ota, flashed through the standard a/b process.