Both of these tutorials are similar, in that you need to boot from a (recent) Manjaro Installer, but they achieve different outcomes.
The first (as the title suggests) will restore GRUB;
- GRUB/Restore the GRUB Bootloader (Manjaro Wiki)
The second, enters a chroot environment (manjaro-chroot -a) with the hope of overcoming a possibly failed update;
Remember that these are tutorials – not step by step instructions – you will be required to make choices according to your needs.
I hope these are helpful.
Regards.
I’ll take this opportunity to suggest an alternative to physically writing a Manjaro ISO to random USBs (which is often prone to failure); With Ventoy you only need to write a Ventoy USB – once – and simply copy or drag any ISO files to the USB.
Rather than reinvent the wheel, read on…