Email privacy services, really confidential or just advertising garbage

Both theoretically and practically, how exactly could an email service (tutanota, proton, etc) access your conversations, when they boast that they offer advanced encryption and that "even they can't read your messages"?

Hypothetical question: If a court comes and decides that Tutanota or Proton give the authorities access to x's email account, how exactly would they proceed to achieve this? Suddenly the encryption disappears, suddenly that quote from their descriptions on the Internet "even we can't see your emails" is …

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