Yes the backplate and carrier are seperate. What type of upright did you get because the disc and drum ones have the thick outer plate orientated differently.

This is the drum setup that I'll be using. As you can see the backplate will be sandwiched between the carrier and the upright. Then the driveshaft goes through that and the splined flange is slid onto the driveshaft and a nut spun on to hold it all together.

There is also two types of shaft,push in and "Lobro" bolt in type.
As it suggests one just pushes into diff and outer hub end and the other bolt togther with the shaft in the center held in with 6 alan/cap screws at either end,as others have said the carriers are different and if not fitted in the right place will let the caliper interfere with shock mount.