From what I've heard
From what I've heard, and this is probably third hand info at best... Best Buy used to have it's own depot for fixing at least Apple stuff, probably other things as well, but as I understand it, Apple has decreed that everyone has to start moving to a store based system where basically you trade repair time for repair quality, because most store level techs have no business being within 10ft of a screwdriver, let alone working on someone's computer. At least in my experience, and that's not to say there aren't some people with real skill working at the store level, but they're often harder to come by. It also doesn't guarantee the Apple tech or techs at their depot are any better than the store level ones, but usually just out of sheer volume of experience (having all the Best Buy's across the country sending units in) they're going to be in a whole other league.
Most retail chains will offer you a loaner if you have one of their store contracts/warranties, but if you just take it in there as a walk-in type repair, then not so much.
I have serious doubts that Apple actually does many repairs in-house in their own stores. They're just too busy swapping out bad iPhone/Pads/Pods every time I've been in one, and I ask fairly simple questions for any ACMT and they give me a deer in headlights look. And of course Apple retail stores don't have to play by the same rules everyone else in Apple's repair network does, so things like Apple corporate picking up the cost of 2nd day airing a unit to a repair depot makes for a really unfair advantage when you figure just how much that would cost a retail chain like Best Buy, Fry's, and any others that do their own repairs. It'd probably be millions of dollars annually.
Anyway, all I can really speak to is the Apple end of things, because that's what I do and what I pay attention to. No other vendor seems to give a gerbil's rear about what some repair place does with their units. Apple is very particular, and a holy terror to deal with on the business side. Just think about your sort of typical snooty butler, hotel concierge, restaurant maitre d, and then drop even the sarcastic attempt at being polite.