This paper studies the homotopy theory of the Grothendieck construction using model
categories and semimodel categories, provides a unifying framework for the homotopy
theory of operads and their algebras and modules, and uses this framework to produce
model structures, rectification results, and properness results in new settings. In
contrast to previous authors, we begin with a global (semi)model structure on
the Grothendieck construction and induce (semi)model structures on the
base and fibers. In our companion paper Batanin et al. (2025) published in
Theory and Applications of Categories, we show how to produce such global
model structures in general settings. Applications include (commutative)
monoids and their modules, numerous flavors of operads encoded by polynomial
monads and substitudes (symmetric, nonsymmetric, cyclic, modular, higher
operads, dioperads, properads, and PROPs), and twisted modular operads. We
also prove a general result for upgrading a semimodel structure to a model
structure.