How to shop in CEX?

Excuse the extremely beginners questions but as someone returning to gaming and wanting to buy cheaper titles in CEX can someone explain how it works?

Are the cases on the shelves the actual ones you get? So you choose one and take that to the counter and they put in the game and manual(?)

If you want something in a case do you queue up at the same counter then ask for it?

If you’re trading in something for credit is it the same counter where you buy things?