In the ARM world, there isn't even a standard way to boot, and there are no standard hardware interfaces - except maybe the interrupt controller, since it's part of the CPU and only ARM designs the CPUs.

On any PC, you can still use BIOS/UEFI services to get a basic framebuffer and keyboard input. You cannot do that on embedded ARM devices - you need to get several layers into the graphics stack to have a framebuffer. I tried it on the PinePhone, using existing source code as a reference, and the furthest I got was sending commands from the video port to the LCD controller and then not having an oscilloscope to see if the LCD controller replied back.

I worked with ARM boards, I know a bit about it. Booting into Linux is never hard, it's all about using uboot, sometimes with tiny patches on top. I think it's actually even easier with android phones, as you don't have access to the low level bootloader, you just use fastboot stuff.

Having basic framebuffer in BIOS/UEFI is neat for toy OSes, but not very relevant for something practical. You gotta need proper driver for GPU. And if you're just starting, UART console is actually more preferable way to interact with board, IMO.