The other Austal ships are Aluminum. How are these things armored?
They are not. No modern ship is.
They are compartmentalized. A 500lb explosive, if it hits, can take out the superstructure of most modern vessels.
They are designed to fight at sea from dozens to hundreds of miles apart...
However, a BB can take a hit from anything but a nuke a keep on pumping 2000lb shells at the beach.