Some silly noobie questions:

I know the AR-15 platform lower can take a range of ammo sizes, but what is the difference in the magazines?

What is different between the .223/5.56 and a 6.8 SPC II? Are there any calibers that can use the same .223 mag besides 5.56?

If the exterior of the Mag is the same for all calibers, what changes on the interior? I am guessing spring and follower for one, but what else?