You mean, directly? I remember seeing a few download links for some images, but many of them aren't directly available (random example: [0])

But otherwise yes, it's one of the best online archive. Wikimedia & some museums's websites are good sources of high-quality material as well.

[0]: https://artsandculture.google.com/asset/nicolaas-rubens-wear...

many of the high res images on wikipedia/wikimedia of notable paintings come from this google project (the category for it has ~25,000 https://commons.wikimedia.org/wiki/Category:Files_from_Googl... )

The super high resolution photos are taken with a special rig. Details: https://www.dpreview.com/news/1947958268/google-art-camera-u...

From what I recal, the software used for stitching is more like Excel than Photoshop.