The CPU gets the i/o completion for the read, and is in charge of the ram address where it was stored, but it doesn't need to read that data...

Modern NICs use packet descriptors that allow you to more or less say take N bytes from this address, then M bytes from some other address, etc to form the packet. So the kernel is going to make the tcp/ip header, and then tell the nic to send that with the next bytes of data (and mark it for TLS however that's done).