@quinn
I'm not an expert on Niantic's internal operations but I am an IT guy who has played Ingress since its initial release so I can share some observations.
One problem of geo-spatial mapping that Niantic tackled early was location drift in heavily built areas. GPS works great in open areas but less so where satellite signals are impeded or reflected by buildings. Modern smartphones combine WiFi hotspot detection with GPS to improve location accuracy but needs "precise location" to be active which consumes more power. The one other type of mobile app that does this is map/navigation apps, but those are mostly used while stationary or on transport, and rarely while walking. Niantic game mechanics push players to walk around places of interest with the game active so Niantic can collect higher quality location data in places where the data quality tends to be poor.
The next breakthrough was the addition of Augmented Reality (AR) APIs to Android and iOS. Prior to this Niantic had players take photos of landmarks, but with AR-capable phones Niantic added 3D scanning of landmarks. Not as many players do this but it provides richer data and can even map indoor areas.
When I started 3D scanning I thought Niantic was only mapping the landmark, but eventually realised they were also mapping the background to determine where the landmark was in relation to nearby landmarks.