A battery that is perfectly good why when it is -30C
won't it start a vehicle.
Please note we are referring to a perfectly good battery
and a totally functioning car.
What is happening to the battery that it cannot crank
the starter fast enough.
Chemically something is happening inside the battery.
Is it the electrolyte that freezes and chemically do what
it should do? Does something happen to the lead plates?
Does the 12 volts drop to a lower voltage? Does the current
output of the battery drop?

The cold is doing something to the battery What Is That
Something?
Well informed auto mechanics please step up to the plate!

I do not need to start my vehicle my spare battery connected
in parallel with the frozen battery does that.

If your frozen battery won't start your vehicle and you remove
it and take it indoors and let it heat up to house temperature,
after reinstall it on the vehicle' it will then start the vehicle.

What is happening to the battery internally at -30C?

Denny B