I’m guessing that the retros just slow it down enough to bring it to a low enough orbit that atmospheric friction will do the rest of the job. True? If so, just how much does it slow it down, and to about what “altitude?”
I was at the KSC monday and listened to an astronaut lecture. IIRC the shuttle only slows by about 300mph from 17,500 to start reentry.
If you want to know How Stuff Works, there’s plenty of places to look.