Grub doesn't grok RAID, so all files that Grub accesses (itself, kernels, initrd images..) must all be where it can find them without having to understand RAID.

For RAID1, this can be simple, but often isn't.

The usual method is a static non-RAID boot partition, that is present (and bit-for-bit identical) and on all drives.

Normally Grub would just boot from the first drive, but if that drive dies, then one can tell it to try the next drive.