Good question. I can explain. "storage.googleapis.com" is an endpoint for Google Cloud Storage's "XML" API. Google Cloud Storage (GCS) is a cloud blob storage service similar to Amazon S3.
This particular API, the "XML" API, is designed to be API-compatible with S3. The XML namespace, 'http://doc.s3.amazonaws.com/2006-03-01', is therefore the same. This allows third party tools like 'boto' and the like which work with S3 to work with GCS with only a switch in hostname.
I have no idea. I'm not sure what you were looking at.
If you visit just "https://noto-website.storage.googleapis.com/", that's a request to list all of the objects in the bucket, so you'll see an XML document with that namespace and the results of a listing.
If you were trying to download a specific object from that bucket, you'd either see the resource itself, or you'd see an XML error result of some sort (404, 403, etc). So I assume that either you mistyped the object name once or else the object had been deleted for some reason.
This particular API, the "XML" API, is designed to be API-compatible with S3. The XML namespace, 'http://doc.s3.amazonaws.com/2006-03-01', is therefore the same. This allows third party tools like 'boto' and the like which work with S3 to work with GCS with only a switch in hostname.