Historically a huge amount of effort went into making PC hardware enumerable at runtime and hotpluggable: BIOS, then ISA PnP, then PCI/PCIe, and ACPI. eg. There are BIOS functions dedicated to telling you, in no uncertain terms, what RAM you have and the addresses it appears at.

ARM's architecture is still quite firmly stuck with the embedded approach where you get your memory map and peripheral availability at compile time, probably by reading addresses from a manual published by your SoC manufacturer. Usually at early boot there is nothing which will tell you where your RAM is, what peripherals you have and where they are, etc. You either just have to know, or have some non-standard configuration mechanism which tells you.

Things are improving, but slowly. ACPI exists for ARM and is being actively worked on, but isn't widely deployed yet.