Aircraft and automotive industries face increasing needs in generating large and complex simulation models, especially at the level of assemblies, sub-systems of complex products. Starting from the digital representation of sub-systems, i.e., digital mock-ups (DMU), as available from CAD software, the major steps of the simulation model generation are described. This incorporates the geometry analysis of the DMU to derive functional information. Subsequently, this information is used to perform model simplifications and domain decomposition consistently with the simulation objectives. Given the complexity of these models, the domain decomposition is a key issue to adaptive simulations to be coupled with COFAST software as well as error estimators using the LATIN method to avoid solving large systems and to take advantage of their decoupling capabilities. An assembly of bolted components illustrates the proposed approach.