I’m curious about how they are building the maps – by flying drones about in grid patterns, and avoiding objects in their path?

Regardless, Airmap is one of the first mapping projects for enabling drone travel:

• real time weather
• airports
• controlled airspace maps
• dynamic information like wildfire temporary flight restrictions

Ultimately it is permission-based. You can go here, and can’t go here, so work out the best path. And it could dynamically change.