How would we know if this is true? For instance, is the source code subject to independent review, with verifiable builds made from that source so that all depl

How would we know if this is true? For instance, is the source code subject to independent review, with verifiable builds made from that source so that all deployments have to show matching hashes? Or is more of a pinky-swear kind of a thing? If so, is the pinky swear even documented somewhere?