Someone else mentioned the Arch wiki under "Arch boot process" specifically. Since you mentioned GRUB, though, also check out the Arch wiki's article on GRUB: https://wiki.archlinux.org/title/GRUB

I found it invaluable when debugging some issues a few years back. Try unpacking the initramfs and reading the scripts contained in there, too: https://wiki.archlinux.org/title/Arch_boot_process#initramfs