Re-Install after failed upgrade

Hello together,

I recently bought an old MacMini which was already OpenLegacyPatcher-patched ;-).

Unfortunately, I started the system-upgrade process of MacOS (and I couldn't stop it) and after reboot, I can't use my usb devices anymore or there may be something else preventing me from entering my login informations (Mouse/Keyboard don't react).

Long story short: I want to reinstall.

Question: which Volume has to be formatted?

I get offered "Install MacOS Montery", 50GB (36 available) and "Mac OS Start" (450GB, 428 available).
The first one say protected ("geschützt") and the latter says it is not possible to "revert" ("Zurücksetzen des Volumes nicht möglich").

In the disk utility there are

- Install macOS Montery

- Volumes on Mac OS Start

-- Mac OS Start

-- Mac OS Start - Daten

Can you point me to the right direction?

Thanks in advance,
pheidrias