debian-installer RAID question

[Felix Natter <fnatter@gmx.net>]

hello fellow Debian users,

I have an SSD for the root filesystem, and two HDDs using RAID1 for
/storage running Debian10. Now I need a plan B in case the upgrade
fails.

So I made an experiment with a VM and rougly the same setup (disk-wise),
and found out that when reinstalling Debian11, the d-i does recognize
the RAID1 (/storage) and can reuse it while keeping the data.

My question is: How does d-i know how the individual HDDs were combined
into a RAID1? For all that "sudo fdisk -l" shows, the disks are
"Linux raid autodetect". For all I see, it could be a RAIDX, X!=1 or
two different RAIDs .... Are there RAID headers on the partitions?

The same thing applies when I boot a GNU/Linux rescue system: I think I
can mount the RAID1 if I know the member partitions and type of RAID
using mdadm?

Many Thanks and Best Regards,
-- 
Felix Natter
debian/rules!