Most spas are made of a single sheet of "plastic" (usually acrylic), that's heated and sucked down into a mold (called vacuum forming).
Rotomolded, AKA rotational molded, is a rotating mold, into which plastic pellets (I believe typically polyethylene) are put, then heated till liquid, and the mold rotates to evenly cover the sides. It's then cooled and removed from the mold.