The simulation framework for coupled shallow and non-shallow free-surface flows was introduced in Mintgen 2017, where the depth-averaged shallow water equations (SWEs) defined on a two-dimensional (2D) region are coupled with a three-dimensional (3D) Reynolds-Averaged Navier-Stokes (RANS) solver employing Volume-of-Fluid (VOF) free-surface tracking. This framework was originally implemented within the foam-extend 3.1 distribution of OpenFOAM. Such a coupling strategy provides an efficient and accurate approach for multiscale hydraulic and environmental flow problems, enabling regions of practical interest, such as those required to resolve hydrodynamic loads on structures, to be treated using a full 3D formulation, while large surrounding areas are efficiently resolved using the SWEs. The bi-directional coupling is realised through specialised explicit interface boundary conditions. The coupled solver, referred to as shallowInterFoam in Mintgen 2017, is re-implemented in the modern, multi-region, multi-physics open-source framework multiRegionFoam (currently developed as multiPhysicsFoam) (Alkafri 2025). This framework offers enhanced scalability and facilitates the modular extension of physical models and solvers, including processes such as subsurface transport and atmospheric interactions. In the present work, coupling between physics-specific regions is achieved through interface boundary conditions defined at the region interfaces. To enable simulations on realistic terrains, a new 2D-3D mapping method for non-conformal meshes at the region interface is developed, removing the conformal mesh restriction present in the original implementation. This allows complex topographies to be represented accurately and flexibly across coupled regions. Beyond architectural improvements, several physical process extensions are introduced. Some of these include precipitation source terms in both the shallow water and three-dimensional RANS formulations, as well as soil infiltration models to account for surface-subsurface mass exchange. Together, these additions broaden the applicability of the coupled framework to rainfall-driven overland flow, flood modelling, and hydrological interaction studies.