Don't recognize the term "spring seat" in this context.
The spring sits on the top surface of the head and the retainer
sits on top of the spring. (The seal goes on before the spring and
the retainer.)
The keepers (collets) hold the retainer to the end of the valve making
it compress the spring. But to get the keepers in there you have to compress
the spring, with the retainer against it, far enough that you can get the
keepers into their grooves in the end of the valve. Then you gradually let
the retainer up around the keepers and the tapered hole in the retainer fits
the taper on the outside of the keepers and holds them tight against the
valve stem.

(Caution: possible info overload ahead!) If you get fancy double valve
springs you may have to machine special spring seats into the head for them.
This should NOT be a problem for you.
--
George Downs Bartlesville, Heart of the USA!