Buildings are buyable on the side of your plot and are unique to each plot. They gives you other services or boost than other items.
Refining your $cOIL is mandatory in order to obtain $OIL. If you complete the whole process you can go down to 3% claim fees instead of 10% at State 1.
Refining takes you time but reduce your claim fees as followed :
When you refine your $cOIL, your tokens are locked in the refinery during the first 24 hours