The disc backing plate is riveted to the drive splines for the hub in many places.
The floater plate that goes in between the clutch discs has drive lugs that engage in the flywheel. The lugs are undercut to give debris that might cause the floater to hang up a place to go. The floater is also decently thick to resist warping.
These are the drive luges for the floater cut into the flywheel.
First, one disc is put down, then the sprung drive hub.
Then the floater is put down with its lugs in the flywheel drive lugs. Then the second disc is installed on top of that.
Here is a close look at the stack. Note the nice thick floater plate. This will really resist warping.