Pretty much similar to a basement car park.Pardon my ignorance but Id love to hear a bit about this one, I don't believe I have ever seen a firebunker but they sound like they could be a pretty handy little prep if you lived in bushland with limited roads etc.
Block work that has steel and concrete in fill, or formed concrete walls.
Concrete slab and concrete roof with earth covered over the top.
A fire proofed/rated , door/hatch.
Filtered air conditioning
Wildfire Safety - Fire Bunkers