Cell phone great power & connectivity outdoors, high up away from city, but weak in downstairs corner of house. Help to understand?

Cellular signals have a hard time passing through metal and concrete within the walls of homes (especially lower floors). That’s why you might gain two or three bars of signal by hanging out of an upstairs window. Many buildings also use a wire mesh in their construction called a Faraday cage, particularly in homes with stucco exteriors. This mesh blocks external static electricity fields, meaning that cell phone signals cannot get through.

Because if you are below ground, the ground is going to obstruct the signal.

A place that's high up is more likely to have a clear line of sight to one or more cell towers.