If on the ground, ideally :
A raft or deeper excavation on the edges and maybe intermediate excavations, so the slab has thicker ‘mini-beams’ on the sides and perhaps every three meters or so. Those beams can use reinforcement or rebars
• then geotextile to keep the gravel from embedding in to the ground .
• then gravel.
• then thick plastic to avoid gravel and ground from sucking water from concrete while curing .
• then perhaps steel mesh to control retraction cracks .
• then concrete.
Pour water on the concrete three times per day for the first seven days.
Slab has to be 4 inches minimum, do not use rebars except at edges against other slabs if you pour slabs separated or mini-beams (strip footing).
You can build 3.35 inches (85 mm) slabs if you use what is called a waffle raft slab.
Width of edge ‘beams’ or strip footing is 300 mm (10 inches), depth depends on how reactive the soil is, rebars on the beams can be given by a structural or civil engineer that tests the soil.
Here you have a good raft slab before pouring.
Notice the beams or longitudinal ‘hole’ running along the middle of the picture, with rebars, the gravel shaped with black plastic on top, bricks on the outside to form the edge beams, the separators that keep the rebars in beams at least 10 cm (4 inches) from the ground and act as stirrups (vertical squares along the beam that take shear stress).