If the requesting user is not on the "Allowed" list (Friends, Friends of Friends, etc.), the server simply does not return that data. There is no "secret URL" or "cached version" publicly accessible. The data literally never leaves Facebook’s secure database unless authorized by the privacy rule.
When you set your profile to "Private" (or "Friends Only"), Facebook’s servers do not send the data of your posts, photos, or stories to a user who is not logged in as a friend. The server checks the requesting user's ID against the privacy settings of the target profile. fb private profile viewer
But here is the hard truth: Almost every tool, website, or app promising this service is a scam. If the requesting user is not on the