Best Buy is an AASP (Apple Authorized Service Provider) so the pricing would be identical if we could do it.
1. The XR is not like the newer iPhones, the back glass cannot be swapped without disassembling the entire phone. For this reason, Apple and AASP repairs involve swapping the entire chassis of the phone. This is why the price is what it is.
2. This would be up to you, but that price is honestly cost prohibitive when you compare it's specs with a current iPhone SE in the same price range.
3. We've already discussed the back glass situation. Honestly I would recommend slapping a case over it and living with it as that will push it beyond the price of a newer device. Swapping a battery is possible at a lower price. Again, Best Buy as an AASP would do it but the parts are likely not available to us. The NAND is not socketed and is not upgradeable. It would require replacing the entire PCB, which essentially makes it a new phone.
Caveat to the above: Apple provides parts and tools to AASPs along with training and repair guides. These mostly include part swaps and not board level repairs or modifications. Micro-soldering is very challenging. With that said, you might find 3rd parties offering some of the services you requested if you do an internet search. There are challenges when not using official parts, used parts, and not using the tools to properly calibrate/configure the new items. Some of these include: Loss of TrueTone screen, iOS reporting battery issues and/or incorrect charging percentages, non-functioning TouchID or FaceID.
I am a Best Buy Employee, but the views that I post are my own and not that of Best Buy.
***Know that I am not able to issue official responses and that my posts are only here to help before an official response is offered by a Best Buy Social Media Specialist***
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