In Umbria where I live, olive mills will mill your olives separately from others if you have 400 kilos or more of olives to press. This is vitally important as you want to have your olives from your olives.
When your bring your olives to the mill they are first weighed and then gently threshed so as to remove any leaf and branch material or any other foreign matter. They are then washed and then crushed (whole, olive stone and all) into a paste and this is slowly turned for 40 minutes or more. The paste is then transferred into a centrifuge where it is spun and the oil separated from the solid waste of the olives. The picture below on the left shows the badge of a centrifuge from a local olive mill.
This oil is then transferred to another centrifuge which spins the oil even faster to separate any residue water and solid particles from the oil. The resulting extra virgin olive oil is then poured into your 50 litre tanks for transporting. The picture on the right shows the wonderful freshly pressed olive oil being poured into a container. At this point your oil is unfiltered and you can either leave it to settle and then raked off its sediment or, as I do, you can have it filtered at the olive mill.
Such a simple process. No chemicals no fancy machinery. 100% natural.