I am considering contact lenses for the first time, but don't understand something...?

I just got my first ever pair of corrective single vision eyeglasses (for nearsightedness and a little astigmatism). They work great and I can see things in the distance, further than 10 feet in particular, with incredible clarity! For things closer than 4 feet, I can see perfectly without any corrective lenses, so my new glasses have to come off for computer or reading work for instance. My doctor explained this would happen, and for close and distant vision in one pair of glasses he explained all about progressive lenses to me.

How can so many people wear contacts for myopia, and NOT have the contacts affect their normal and good natural close vision at the same time? How can "single vision contact lenses" do this without being multifocal, as with progressive type eyeglasses?

Is only part of the contact lens Rx'd? How come contact lenses are different from single vision eyeglasses in this regard?