Is Snapchat not encrypted at all? If it's encrypted between the phone and the server then it must have been intercepted at the server end, either by Snapchat or by the intelligence services. Or on the phone before encryption, by the OS, keyboard, or Snapchat app.

Thats not how that works. Even tho snapchat messages might not be e2ee they are still client to server encrypted. The operator of a wifi network can never see your message content, only which services you are using.