virodoran
I do think it wasn't until after Pokemon Go launched and they saw the success of it, that they shifted focus to be more of a platform for these types of experiences (see Niantic Lightship). Additionally, I think Unity offered them the opportunity to integrate with ARCore and collect much more detailed data than they would've ever been able to do on the old Ingress engine. In fact, I expect a significant chunk of ARCore functionality was added specifically thanks to Niantic and Unity (in fact, you see Unity mentioned all over the Google Developer docs for it).
Training data is people taking dedicated video of locations. Only ARCore supported devices can submit data as well. So I assume along with the video they're also collecting a good chunk of other data such as depth maps, accelerometer, gyrometer, magnetometer data, GPS, and more.
[1] https://twitter.com/tinkersec/status/1580722789245280257