I have recently read that many Mac users have success with either of Tab key, or F8.I read that I'm supposed to press either esc or shift on boot but nothing is happening
You are barking up the wrong tree trying to find a correct grub.cfg to edit.
Your actual grub.cfg file will be the one in /boot/grub, not in the efi paths at all.
It will start by saying
If you have the knowledge to safely edit grub.cfg, the changes will only last until the next kernel upgrade, typically, and then wiped.#
# DO NOT EDIT THIS FILE
# It is automatically generated by grub-mkconfig using templates
# from /etc/grub.d and settings from /etc/default/grub
The file you want to edit is /etc/default/grub
Mine, on Deepin, looks a little like this
[email protected]:~$ cat /etc/default/grub # Written by com.deepin.daemon.Grub2 DEEPIN_GFXMODE_DETECT=2 GRUB_CMDLINE_LINUX="" GRUB_CMDLINE_LINUX_DEFAULT="nosplash noquiet" GRUB_DEFAULT=saved GRUB_DISTRIBUTOR="`/usr/bin/lsb_release -d -s 2>/dev/null || echo Deepin`" GRUB_GFXMODE=1920x1080,1680x1050,1600x1024,1400x1050,1600x900,1280x1024,1440x900,1400x900,1280x960,1440x810,1368x768,1360x768,1280x800,1152x864,1280x720,1024x768,auto GRUB_SAVEDEFAULT=true GRUB_THEME=/boot/grub/themes/deepin-fallback/theme.txt GRUB_TIMEOUT=10
You can edit /etc/default/grub from a USB stick and save it, but then in order for the effects to take place, you still have to run
You can't do that from the stick without using chroot.
Can you give us the output for /etc/default/grub via the USB stick?