It's free in the sense, that you can browse around, but if you wanted to do some analysis on it, you would need access to the raw bulk data, and this is very expensive.

There is free aerial imagery data available (e.g. ESA's Sentinel-2), but it has lower resolution (10m/Pixel). If you want to do the 3D building thing, I estimate you need data with something like 1m/Pixel and from different perspectives. This data volume is rather big (Exabyte-scale) and it is non-trivial to host and access it, so currently only the big players have it.