How do you change the gasket on the oil filter housing and is there anything that is behind it?

The gaskets on most oil filters is bonded to it and, while it may be possible replace it, it's probably cheaper and certainly easier just to buy a new filter with it's own new gasket attached.

If by 'oil filter housing' what is meant is what the oil filter is attached to on the engine, usually this is machined into the engine block and there is no gasket to remove unless the oil filter gasket has unbonded from the filter and stuck to the block.

depending on vehicle. I have a for 2003 Ford Exp. and it is mounted with 4 bolts. on the same mount there is an approx 2" hose ,coolant presumably, that attaches as well.

On a Jeep 4.0L for example, you use a t60 torx to remove the center bolt. You then replace the three o-rings and reinstall.