Look at SW: Rebellion and you have a quite good impression what awaits you!
If a planet turns its affiliation, the most buildings remain intact. So the production facilities are on your disposal at once. That's why taking control of planets with good infrastructure is not a bad idea.
The economy mechanisms differ significantly from the ones in SW:EaW. Mines produce raw material, which refineries convert to refined material. Refined material can be used to produce buildings and Troops. Every mine-refineries-pair generates a fixed amount of maintenance. All other buildings and units consume maintenance. The efficiency of mines and refineries on a particular planet depends on the population support for your site.