It's no different from any wood floor. Why do you think this is a special case? All wood floors in houses are finished. You're only doing the top surface, right? And it's plywood?

There’s nothing under it but the beams supporting the floor. My house is temperature controlled. My garage isn’t. Just wondering if sealing the top while leaving the bottom exposed to humidity and temperature fluctuations of 0-100 degrees is the right thing to do.

Okay, "standard sub-flooring" means nothing. I'll assume 4x8 sheets even though my subfloors are 1"x8". If it's plywood or OSB, they are both filled with adhesives that impede moisture flow. Every layer of the plywood is separated from its neighbor by a layer of plastic resin that holds it all together, every chip on the OSB is completely surrounded by the same type of plastic resin. There is no wood to wood to wood path in your subfloor for moisture to migrate.

Your subfloors already have the barriers inside them that inhibits moisture migration.

I would seal them with a single coat of water based poly just to make them easier to clean. Not much more than that needed.

Thank you