What symmetric algo does TLS use?
My question is once the key exchanged has taken place what symmetric algo is used?

Well, that is negotiated between the client and the server (and that negotiation takes place along with the key exchange).
TLS 1.2 had a rather long list of symmetric algorithms that the client and server could choose from; in TLS 1.3, they trimmed it back significantly, currently the options are AES-GCM, ChaCha20/Poly1305 and AES-CCM.