It's not like a car, because your person is so much narrower than the structure of a car, less of the rear view is blocked width wise. You just put them towards the outsides of the handlebars and angle them inwards a bit. With careful placement you can have really good awareness of what's going on behind you, both directly and side to side. Oddly not very common on bicycles, but mandated on motorcycles.

I ride a motorcycle and bicycle regularly. I generally don't focus my mirrors on right behind me.

At most, when I'm at a red light, I lean over slightly to make sure the next person is actually going to stop and not rear-end me. Otherwise, while in motion, I have never felt the need for a rearview mirror on any form of two-wheeled vehicle.