As an example what I worked out for 1 of my dogs:
Lies on the lower part of the staircase where it turns and there is a flat larger area if some people in the house are upstairs and others downstairs. I think this is do he can keep an eye on things and know what is going on. He has zero visibility of anyone in that spot but he knows who is where he's in the house and keep track of movements as people walk past/over him.
Lies in front of the oven door pretending to sleep when I'm cooking, prime spot for observing anything dropping on the floor as my prep surface is next to the oven and he can also watch the fridge. Total tripping hazard for me, very dangerous in fact, but I don't have the heart to tell him to move.
Lies by the main door to the garden for observation purposes. Even when the door is wide open. He can lunge into the garden and chase away small animals unaware of his presence. Again dangerous because he is big and sprawling, tripping hazard.
He has 3 very specific spots in the house during hot weather. These don't look obviously cool to me, but I tried the one under one particular bed he uses the most and it is surprisingly cooler (no idea why).
During cold weather, if the sun is out, he goes on one particular bed at a certain time mid-morning where there is a spot of sunlight. After lunch, he has a different sunny spot on the floor in a different room.
The other dog just seems to be in the vicinity of this one but doesn't appear to make choices, just sort of hangs out around the first dog, maybe for company.
So I think there are multiple factors at play.