Refine?
You grow a plant, dry the leaves and smoke - really pretty simple
What’s to refine?
The oil/THC is taken out in solution by a solvent for homemade hash. The solvent is then extracted or evaporated off. From I have read amateur cooks use something like Butane, extremely flammable. One stray spark without adequate ventilation and KABOOOM. Sort of like a half a$$ed Meth lab. It sounds simple but can be very dangerous. They really need a Mr White to do a good job.
I’ve heard that refining the marijuana oil gives you hashish...which can then be sold for a pretty penny.
Value-added product, I’d imagine. Select clientele, fer sher.