The traditional ARM answer is that at a low level and at bootup the details are all very board specific -- what kind of BIOS equivalent you have (if any), where

The traditional ARM answer is that at a low level and at bootup the details are all very board specific -- what kind of BIOS equivalent you have (if any), where it starts you off, whether there's a bootloader like u-boot involved, where devices like the serial port are: all these things vary by board. For 64-bit ARM things are moving a bit more towards a standard setup with UEFI. In general, ARM assumes that the initial boot code will be customised for the specific chip, so unlike x86 which starts out looking like an 8086 until you flip it into protected mode, a 64-bit ARM CPU will start out in 64-bit and you can get at the backwards-compatibility 32-bit support by dropping down from 64-bit to 32. Back-compat is for applications, not OSes and bootloaders :-)