RAM and cpus use the proper 1024 B to kB to MB to GB. Unlike hdd makers that paid ISO to retroactively make it 1000. Or worse internet providers that use bits (b) instead of bytes (B) (reasoning that signal transmission uses bits).

It’s definitely the kernel and bios reserving pieces. They need it to run the computer, after all. Afaik even other hardware can reserve memory for itself, like gpus or network devices.