That used to be true, but ARM has been standardizing these aspects now for a long time. Modern ARM systems can use ACPI for example. Android phones have differ

That used to be true, but ARM has been standardizing these aspects now for a long time. Modern ARM systems can use ACPI for example. Android phones have different builds because the hardware varies a lot, and because the OS is an integrated part of the product offering. It's not like the PC space where the hardware makers can't compete by better integrating the OS.