"Filling the fresh water tank using a hose. Am I correct in thinking that you if you screw the hose onto the filler port that no water goes into the tank?"
That is correct.
"To fill the tank you have to use a something with a spout that doesn't seal (so no pressure)?"
A garden hose end will fit in my tank fill port. I use an 8" hose connected to a garden hose shut off valve. The 8" section came with a RV filter kit. It has a spring around the hose that supports it. The shut off valve came from a local hardware store. I used the plastic RV valve Frank recommended for a while, but it is not robust. It will bend too easily.
"Or is there a valve to turn to a tank fill position?"
My 2018 Kodiak Cub does not have a valve with a fill position.
I wish you good luck and happy trails ahead!