Resources are present on a planet as stockpiles. Any resources that a planet generates which it does not immediately consume or exported will be stockpiled on that planet. Planets will attempt to import or produce slightly more of any resource than is needed to meet current demand, so a stockpile of any resource that a planet produces or consumes will almost always be present.
All planets have a class. Planetary class determines what, if any, life support resources are required by the planet's population and the type of habitat structures that are present and can be built on the planet. In turn, habitat structure type determines maximum planetary population. Planetary class also influences what raw material deposits are present.
All planet classes have deposits of between two and five raw materials. There will typically be some variety in raw material deposit abundance between individual planets of the same class, but all planets of the same class have the same types of deposits.