They look things up constantly, you just don't see it because it's typically very boring and not a good use of time for you or the MD. They get symptoms from you and then will go off to research. Also, a broken arm or strep isn't very difficult to figure out. ~80% of visitations are fairly simple.

The site most MDs use is here: https://www.uptodate.com/home

They look stuff up all the time. Most physicians try to stay on top of developments in their field by reading scientific journals. Most also have access to subscription services which will provide background and lay out current best practice guidelines / treatment recommendations for almost every disease. Another poster mentioned uptodate which is one of the most popular examples. It's not uncommon for a physician to reference this with the patient still in the room.

I've read that med school is really heavy on memorization.

Cynical answer: the culture of medicine was set during a 2000+-year period when physicians harmed more than helped; the way they made a living was by appearing authoritative.