The issue with Arm is they aren’t all one board/chip, you have ARM based design licenced from them and they are built to meet the criteria of what the customer requires. i.e. for my iomega NAS there isn’t firmware boot, you just have to generate an empty section of 00s on the first 32bytes of the drive so the board knows that is the drive to load the kernel from (no grub no uboot) and the board is set to do the rest from the next partition.