Boot loaders and boot managers.
Boot can be in MBR or UEFI mode.
If grub is not installed, install it with
When changing grub2 options in /etc/default/grub, recreate grub config by running
grub2-mkconfig -o /boot/grub2/grub.cfg
UEFI boot requires a /boot/efi partition, knows as EFI System Partition (ESP). ESP is vfat, 500MB, mounted at /boot/efi as recommended by openSuse installer.
Once the partition is created, install grub on it.
To mount a drive from rescue USB, see here.
Also see this for boot troubleshooting.