What -d Actually Sends

The request line told you the method. Now look at the body, which is the last thing in the request, after the blank line.

Whatever you passed to -d is sent literally. -d 'name=ada' puts exactly name=ada on the wire. Several -d flags are joined with &, so -d 'a=1' -d 'b=2' sends a=1&b=2.

curl also adds a Content-Length header counting those bytes, because the server needs to know where the body ends. You do not have to compute it, which is one of the quiet conveniences of using a client instead of nc.

Reading the body back off the wire is the fastest way to settle any argument about what a client is really sending.