If you're curious about where Princeton's curbside recyclables go for separation at a Materials Recovery Facility (MRF), there are a few pictures at the Colgate Paper Stock Company's website. For a more dynamic rendering of the very impressive sorting technologies now being deployed, there are many videos on the web that give virtual tours of various recycling processes. They are likely analogous to the Colgate plant.
Here's a new sorting plant built by Waste Management in Philadelphia.
If you like a tour with a British accent, try this video from Northumberland, UK.