God cards (Yugioh)

I’m confused as to how the god cards work. (Obelisk, Slifer, and Ra) from my understanding they cannot be targeted by monster effects or spells/ traps that directly target them. But field spells work on them. And for example if someone put a swords of reveling light down it would only last one turn on them for my understanding. What would be some example of some cards that work on them? Also am I wrong in how they work?