GRUB loads itself into memory in the following stages:
The UEFI-based platform reads the partition table on the system storage and mounts the EFI System Partition (ESP), a VFAT partition labeled with a particular globally unique identifier (GUID). The ESP contains EFI applications such as bootloaders and utility software, stored in directories specific to software vendors. Viewed from within the Red Hat Enterprise Linux 6.9 file system, the ESP is
/boot/efi/, and EFI software provided by Red Hat is stored in
/boot/efi/EFI/redhat/ directory contains grub.efi, a version of GRUB compiled for the EFI firmware architecture as an EFI application. In the simplest case, the EFI boot manager selects
grub.efi as the default bootloader and reads it into memory.
If the ESP contains other EFI applications, the EFI boot manager might prompt you to select an application to run, rather than load grub.efi automatically.
GRUB determines which operating system or kernel to start, loads it into memory, and transfers control of the machine to that operating system.
Because each vendor maintains its own directory of applications in the ESP, chain loading is not normally necessary on UEFI-based systems. The EFI boot manager can load any of the operating system bootloaders that are present in the ESP.