The cap and the bottle in soft drinks are made of PET. Most deposit schemes will accept plastic (PET), or aluminium and a machine will separate and sort the material into the appropriate bin. Cans get melted down, plastic is stripped, washed, turned into pellets and fed back into hoppers that make new bottles. Because it's all the same plastic material it can be ground up into pellets and fed back into a machine to make new bottles. The biggest issue is probably that caps are usually black, red, blue or whatever so I imagine somewhere in the process the chopped up plastic goes past cameras that sort fragments by colour.