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Description: "On Red Hat Enterprise Linux 9, the initramfs contains an entire usable system by itself."

I have a problem with the above sentence. The initramfs contains a complete systemd instance. I think that's what is meant here. I have an entirely usable system once the system has finished booting.

Also, I don't consider the Boot process diagram accurate enough.
BIOS firmware section:

"Looks for the bootloader in /boot/grub2/ directory"
What do you mean? The bootloader lives in the MBR and the post-MBR gap.

"Loads the grub2 from /boot/grub2/grub.cfg"
That's the config file that grub will read when it is launched from the MBR.

UEFI firmware section:

"Looks for the bootloader in EFI partition" then "Mounts the UEFI partition from /boot/efi". Then says "Loads the grub2 menu from /boot/efi/EFI/redhat/grubx64.efi".

No. The firmware reads the boot entries in the NVRAM to find out which EFI application to launch and from where. In RHEL, this is going to be the shim (needed for secure boot) which then launches grubx64.efi.
The config file is /boot/efi/EFI/redhat/grub.cfg but in RHEL9 this just reads /boot/grub2/grub.cfg now.
The boot entry in the NVRAM could also by a disk. In this case it tries to launch the fallback efi application. This is how UEFI bootable removable media works.