The first OS on laptop was Windows 10. After that, I have added Manjaro. Manjaro has added a Grub menu, with entry's for Manjaro and Windows. Now I want to switch to Arch. I have installed Arch on a separate partition. Arch has not added a entry in this grub menu, but this was expected. I have used update-grub from Manjaro, what added Arch to the menu.

But for some reason I can't edit the menu from within Arch.

I have tried to understand Arch Wiki what I have to do, to edit this menu from Arch. Can someone please help me to get this to work? I have tried to add my EFI partition /dev/sda1 via fstab to /boot/efi like in Manjaro, but this doesn't help to find the same menu with grub-customizer.

Any idea's?