As others here are pointing out, with passive monitoring (i.e. Wireshark) you can't decrypt Perfect Forward Secrecy (PFS) traffic even if the private keys are known / later discovered. This is by design and is the great thing about ephemeral Diffie-Hellman key exchange. The private keys would only allow you to impersonate the server in an active attack, not decrypt a new direct connection.

The key is never transmitted (even in encrypted form), unlike with RSA. A good way of thinking about this is with a colour mixing analogy. I had a go at explaining this (with a Lego stormtrooper) here: https://unop.uk/understanding-encryption-and-key-exchange/