That last one is Monadenia fidelis, the Pacific sideband snail. With the swirl of colours on their shell and the red-and-black tessellations on the body of the animal, easily our prettiest land snail (also one of our largest land snails). They are native, and mostly decomposers (not very interested in eating your garden plants). I’m jealous you have them in your yard.
The black slugs are Arion ater, European furrowed slugs. One of the worst garden pests.
Leave the beautiful snails alone and do not feel the slightest guilt for crushing those slugs underfoot.