Here is an explanation of one way… https://www.youtube.com/watch?v=nVhM3IYMF8o… This is probably the most efficient model you are going to get. It is essentially downloading 1 building at a time from the sketchup warehouse through google earth.
I haven’t seen where google is trying to sell there new building data set, However Cesium https://cesiumjs.org/demos/CyberCity3D.html looks like they might sell a data set similar to the google earth buildings…
Open source is catching on more and more…https://osmbuildings.org/
Google Earth Enterprise has been open sourced…http://geo.t-sciences.com/, but it doesn’t look like it comes with any data…
This looks interesting too…
But I think your best bet is the method shown in the video…
Good luck