How much of the request boday does server get when http handler get invoked?

Say my client is posting a http request, of which the body is 1 TB.
The first time when my server’s http handler get invoked, how much of the request boday has been in my server’s tcp socket receive-q?
From the TCP level, does r.Body.Read(data) just read data from the socket receive-q or triggers more data get transferred from client to server’s socket receive-q?