GRUB
The bootloader, and the one file you actually edit.
GRUB is what loads the kernel on most Linux machines. Day to day it comes down to one file, /etc/default/grub, where GRUB_CMDLINE_LINUX_DEFAULT holds the parameters handed to the kernel at boot.
The thing worth remembering is that editing that file does nothing by itself. GRUB reads a generated config rather than the one you edited, so a change only takes effect once you regenerate it. On Debian that’s update-grub, a wrapper around grub-mkconfig.
That’s how Plymouth gets switched on, and how a machine stops booting into a wall of text.