Another worrying post.
If you have to ask such basic questions, should you be dealing with high pressure and high temperature steam in the first place ? Instant death awaits the unwary (and unqualified).
Anyway ... joining pipe .... It rather depends upon whether the pipes are co-axial or perpendicular. I would recommend a reducer (maybe two reducers) for the former or a reducing tee or weldolet for the latter.
A spool piece is a piece of pipe, commonly flanged.