The paper addresses dynamics modeling methods for mechanical systems, like ground and underwater vehicles, space robots and vehicles, dedicated to complex dynamics analysis as well as to control designs. Demands required to complex systems performance, their work regimes, and also reliability and fast simulation times for dynamics and control actions, require some specializations in modeling and make room for looking for new modeling and control design methods. The paper presents a modeling framework suitable for constrained complex systems modeling and control design. The framework may be applied to fully actuated and underactuated systems, enables using various parameters for system motion descriptions and it works for rigid and flexible system models. The framework applications are presented in selected illustrating examples.