Thanks for the prompt replies.
I had no success with this method, but it is the right track.
fdisk -l output:
# mount /dev/nvme0n1p5 /mnt
# mount /dev/sda2 /mnt/boot/efi
# arch-chroot /mnt
# pacman -S grub efibootmgr os-prober
# grub-install --target=x86_64-efi --efi-directory=/boot/efi...