I replaced my broken Galaxy S4 screen when in Shenzhen in 2015, and bought a replacement screen for Galaxy S3 for my wife (to have a spare I could use to perform the repair myself back home). My repair was dirt-cheap - I paid something like $10 to get a whole new screen assembly and a new back camera (I broke mine), the whole repair done in front of me in under 5 minutes. The extra screen for my wife was much more expensive - ~$50, IIRC. The difference is, with my phone, they kept the broken parts. They presumably replaced the broken glass in it at their own pace, and put it back on the market.
I also saw plenty of work being done on phone components, as well as people unloading and sorting through big bags of broken phones. There's lots of e-waste recycling going on there.
In much the same way that salvage yards are a source of car parts, yet companies like Tesla are trying to stop that.