There's typically a bit of text at the bottom of the screen attributing the image source; a lot of the current high-resolution imagery seems to come from Airbus. Not all the historical imagery seems to have these attributions, which perhaps indicates it some from some database directly owned by google. But scroll through the historical imagery for different areas and you can get an idea of how much of this data is a composite from different sources. The general priority seems to be continuously updating with the newest image data that matches their resolution standards rather than trying to decide on some set best representation of each area (which would be an enormous undertaking even if they weren't updating every few months at this point).