On Liquid Propane tanks, say, the 20 LB tank on my gas grill, what is the
pressure in the full tank and what is the pressure stepped down to on the
low pressure side of the regulator?

How does this differ from regulators used to deliver Propane to
Propane stoves, furnaces, and so forth?

I've heard that the "Outdoor-Use" regulators step down to deliver gas
at 1/2 PSI, and that "Internal -Use" regulators step down to deliver gas
at 2.0 PSI.

I'm wondering how to make an old propane furnace in my shop run on
20 LB cylinders, and need to understand the stepdown of the regulator
and where to get one inexpensively. I already have all of the fittings
to attach the tank directly to the furnace.

Let's leave code and inspectors out of this conversation for now.