This is what I want to build. However, I have no idea what should be under the fire bricks.
Well - I sure hope you DO build this. IT’S AWESOME!
You don’t need anything special under the firebrick (like insulation board or perlite) as you are not trying to conserve heat (like you do in a pizza oven). If I was building this set-up for myself over a solid concrete slab, I would put a 1" layer of sand between the concrete slab and the firebrick. This way, the sand will act as a buffer between the firebrick and concrete slab - and the majority of the heat will stay in the firebrick. Your concrete slab will get warm / hot - but nowhere near hot enough to damage the concrete slab.
PLEASE take LOTS of pictures when you build this A-Team Grill!
It’s going to be spectacular!