Hills add time because you lose more time going up than you gain coming down. It takes you longer to go up than down.

They add to your total energy expenditure (1 mile w/ elevation gain costs more energy than 1 mile w/o). How much depends on how you pace yourself. If time isn't an issue you can level the hills (eg ride the same power going up as on flat ground) but this is slow, you spend more time riding at a constant output.

If you don't level the hills, then you're putting more energy out more quickly. Maybe you're burning matches, maybe you're producing big peak forces (mashing) that fatigue you more quickly.

I think hills add difficulty pretty much no matter how you define "difficulty."

But you don't need to do anything special to train for them. A watt is a watt, cycling is an aerobic endurance sport. Train to be able to ride long and hard-when-needed.