This is using Neural Radiance Fields (aka NeRF) that first appeared about two years ago: https://www.matthewtancik.com/nerf

Originally, it took more than 1 GPU-week to train a single static scene. After multiple rounds of algorithmic optimizations, it's now practical to train & render whole city blocks, as seen in the Waymo demo.

The best part of it, is the answer to the classic question: "how many polygons?". Because the answer is zero.