Section 12.15.7, “Geometry Property Functions”, discusses several functions that construct new geometries from existing ones. See that section for descriptions of these functions:
Envelope(g)
g
StartPoint(ls)
ls
EndPoint(ls)
PointN(ls,N)
N
ExteriorRing(poly)
poly
InteriorRingN(poly,N)
GeometryN(gc,N)
gc