Good question but not that many people know the answer
Firstly the material is taken to a processing site. Speed here is vital as these sites have very strict limits of what they can hold onsite and will be fined heavily for any breaches. So if you can imagine waste arriving at one end of the site and process is designed to mechanically remove as much as possible that can be sent for more specific recycling. The challenge is just the pure volume of waste as a society we produce.
Ideally everything should be recycled but currently that is not the case and the residual waste ends up going for landfill which is costly and is ever increasing and will continue to do so. Additionally there are less and less land fill sites.
Once the waste has arrived it is conveyed throughout the system at a fairly high speed through the following steps Please remember the machinery I refer to is industrial size able to handle 120tonnes per hour.
All material shredded to around 20mm - roughly thumbnail size as this both de-bulk’s and makes other processes faster.
Material convey via a magnet to remove metal for onward recycling.
The material then continues to next machine which is in essence a magnet but for aluminium again this goes for onward recycling.
Next the material is conveyed on to optical sorters which use a mixture of density and lasers to pick out green, brown and clear glass again this material.
At this stage any wood is also removed which again can be recycled usually as waste derived fuel.
Now the process becomes more difficult. Usually next would be water tanks and this separates items that float or sink. for example plastics mainly float and paper and cardboard sink. The floating material is then scalped and in most cases ends up in landfill and the paper and cardboard is dried and recycled
