I heard that the ancestors of Thais came from southern China.

And Thailand and Laos are ethnically the same?

I heard that the language and culture are similar.

thai and zhuang has some mutual intelligibility

>>213413604 (OP)
As I understand it they both descend from tai-kadai primarily, then less so from austroasiatic migrations in the Neolithic and hoabinhian people before that, but the evidence might have changed
This is a question for /his/