ᐅ What preparations are necessary for constructing a concrete slab foundation?

Hello everyone, I have the following question: what preparations are necessary before pouring a concrete slab? Do the construction companies handle this, or does it need to be contracted separately? For example, excavation, compaction of the subsoil, possibly a gravel base, etc. Thank you very much!

Hello House Building Blog, this is about a foundation slab for a wooden single-family house, 1.5 stories high with a footprint of approximately 10 by 9 meters (33 by 30 feet). The ground at the rear part consists of quality topsoil covered with grass and hazel bushes; at the front part there is an asphalt surface with a gravel base underneath. As an extra detail, there is a non-drainage septic tank made of concrete rings with a volume of about 2 cubic meters (70 cubic feet) in the grassy area, which has never been used (so it is mostly empty except for a few centimeters of water). This tank will likely need to be filled in or possibly removed(?).

Best regards, Till