Hi Don.
Some fluid heads most certainly do have fluid in them, I had a cheaper all in one fluid head video tripod (can’t remember the brand, something along the lines of a Velbon) and the fluid leaked out on a very hot day, made a right bloody mess!
It was a sort of tacky slippery icky stuff!
I also have a Manfrotto fluid video head which I bought to replace the cheap one, I believe this does have fluid in it, this one has never given any trouble, hot or cold. I think the freezing point of the “fluid” is very low, (like might freeze in Antarctica low) it does however get more viscous as the temperature drops though not to the point of stopping working In any conditions I have encountered.
I have no idea if all fluid heads contain fluid.
Cheers, Graham.