it's called laminated flooring

the material is either MDF or recycled rubber. if you want to save cost, use 12mm waterproof plywoods instead

and as for the kalebet u keep saying, it's actually culvert

if you need water supply, just use a 90-deg TEE and connect it from any existing pipeline that belongs to you (or your neighbor)

you'd also need some form of ventilation, so a 2-fan system will be the easiest. you'd need another ventilation fan inside the toilet for obvious reasons

you will need noise insulation for the upper deck. when it rains, it will be quite deafening inside

the easiest way to insulate is to use 2" rock wool and fix it to the underside of the upperdeck with 2" chicken wire mesh (easily available). weld some 3" steel rod so u can tie the chicken wire mesh. it will look ugly, so cover it up with 9mm plywoods.

good luck.