iphone 8 plus backglass cracked

How was the case cracked?

Does the phone still work?

Is the iPhone still under warranty, either regular or AppleCare? If it is it may be covered (see linked Apple document).

If all avenues of Apple fixing it are exhausted and it still works, you can get a case to protect the phone.

Make an appointment at the genius bar of your local Apple Store. You'll have to pay for an out of warranty replacement phone.

iPhone Service Pricing - Apple Support