Grub is the best knwon boot loader. The first step is always to chroot into the installed system. See Chroot from Live System

Manjaro comes with some tools that ease this task.

pamac install manjaro-tools-base
manjaro-chroot -a
#grub-probe: error: cannot find a GRUB drive for /dev/sdb1.  Check your device.map.
#grub-probe: error: cannot find a GRUB drive for /dev/sdb1.  Check your device.map.
#==> Mounting (ManjaroLinux) [/dev/nvme0n1p5]
# --> mount: [/mnt]
# --> mount: [/mnt/boot/efi]
# Changed to installed system
pacman -Syu grub
#[...]
#Packages (1) install-grub-2:2.12-3
#
#Total Download Size:   0.01 MiB
#Total Installed Size:  0.01 MiB
#
#:: Proceed with installation? [Y/n] y
#[...]
# EFI system
grub-install --target=x86_64-efi --efi-directory=/boot/efi --bootloader-id=manjaro --recheck
#Installing for x86_64-efi platform.
#Installation finished. No error reported.
grub-mkconfig -o /boot/grub/grub.cfg
#Generating grub configuration file ...
#Found theme: /usr/share/grub/themes/manjaro/theme.txt
#Found linux image: /boot/vmlinuz-6.6-x86_64
#Found initrd image: /boot/amd-ucode.img /boot/initramfs-6.6-x86_64.img
#[...]

Find the system.

sudo wipefs /dev/nvme0n1p5
# DEVICE    OFFSET TYPE UUID                                 LABEL
# nvme0n1p5 0x438  ext4 3899a753-aaaa-46e0-b034-cccccccccccc 
sudo mount /dev/nvme0n1p5 /mnt

Mount special filesystems:

sudo mount -t proc proc /mnt/proc
sudo mount -t sysfs /sys /mnt/sys
sudo mount --bind /dev /mnt/dev
sudo mount --bind /dev/pts /mnt/dev/pts

Change into the installed system:

chroot /mnt

Unmount the installed system:

sudo umount -r /mnt/dev
sudo umount -r /mnt/dev/pts
sudo umount -r /mnt/proc
sudo umount -r /mnt/sys
sudo umount -r /mnt