Will i get a loaner phone while my phone is being sent in for repairs?

You have to specifically ask for a loaner and sign the agreement -> https://www.apple.com/legal/sales-support/applecare/iphoneloancaen/

But with the back glass broken, they are only going to offer you and out of warranty replacement so they will swap it for your replacement right there in the store.

Usually no, but it depends on the Apple Store.

But... did you make an appointment at that Apple Store explaining your issue? To my knowledge, if the back is broken, they would offer a replacement unit at a convenient price, or, if you have AppleCare+ protection plan, they would charge you $99.

Regards

Giulio

The rear glass can not be repaired and the device must be replaced.

I've never heard of loaner devices.