The document is digitally signed by the publisher, using the publisher's own private key on the publisher's own server. This signature is then verified by your browser on the other end and verified against a CA issued certificate. The intermediaries don't matter in terms of the content, they are just a pipe at this point that can optimize network paths and allow for prefetching of the bytes.
The specification only allows a short lifetime for signed documents (7 days maximum, configurable to be shorter by the publisher), preventing long-lived caching drift. Refreshing will reload from the origin directly.