Download the .deb from their downloads page and run it, just like you would either a .exe on Windows. Their instructions list that as an option further down on the page. Should be higher up imo

Seconded. Love OMV

Passkeys are asymmetric, meaning that the server only ever sees your public key. If the server gets breached, then only your public key is leaked, which isn’t a big deal. Functionally, it’s almost identical to SSH keys.