If I took my phone to be repaired at a phone store instead of Apple, could Apple still fix the back glass or replace it?

If you're looking for a 2nd opinion. You can try, but Apple will almost NEVER touch a phone which has been tampered with by anyone other than Apple or an actual Apple Authorized Service Provider. And having done so, will make it impossible to trade the phone into apple and greatly reduce its value if you try to sell it when you're ready to get a new iPhone.

This was an unfortunate decision to let someone else replace parts on your iPhone.

Apple will refuse to work on the phone if they detect any evidence of tampering by an unauthorized shop.

If you have the time, I suppose it would not hurt to try, but Apple is extremely strict on matters like this.

The best answer may depend on how old it is and how much you are willing to pay for the repair. Set up an appointment at a near by Apple Store to have it looked at. They will not charge you to look at it.

How to make a Genius Bar appointment on iPhone and iPad using the Apple Support app

1. Open the Apple Support app on an iOS or iPadOS device.
2. Tap the device you're having issues with.
3. Select your issue from the categories list.
4. Find the screwdriver icon and tap Find Authorized Locations.
5. Choose a store to visit.