What symmetric algo does TLS use? So my understanding of TLS is that it uses asymmetric cryptography such as DH to perform a key exchange, once this has occurred it then used an symmetric algorithm for the data exchange. (Obviously I am missing a lot of detail here and it may be slightly inaccurate but I think thats generally the high level idea?)
My question is once the key exchanged has taken place what symmetric algo is used? AES? Or is it implementation specific? Or developers preference???
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.