Little bit of both. Pi still uses a sort of unique boot sequence due to it’s heritage. Most devices will have the CPU load the bootloader and then have the OS bring up the GPU. Pi sort of inverts this, having the GPU leading the charge with the CPU held at reset until after the GPU has finished it’s boot sequence.

Once you get into the CPU though the Aarch64 registers become more standardized. You still have drivers and such to worry about and differing memory offsets for the peripherals - but since you have the kernel running it’s easier to kind of poke around until you find it. Pi 5 added someone complexity to this with the RP1 South Bridge which adds another layer of abstraction.

Hopefully that all makes sense. Basically the Pi itself is backwards while everything else should conform. It’s not Arm specific, but how the Pi does things.