Robust numerical groundwater models are useful tools for predicting aquifer behavior under increasing water stress and usage conflicts. In this work, we present the technical development of current numerical groundwater flow modelling for the aquifer used for the drinking water supply for the city of Görlitz in in framework of the project CRossWATER, which is funded by the EU-Interreg-Poland-Saxony-Programme, highlighting its evolution from a basic steady-state setup to an advanced transient simulation framework using MODFLOW 6. Initial modelling began with a steady-state MODFLOW-NWT model based on limited geological detail and conventional boundary conditions. The model was later converted to a transient configuration, with monthly stress periods and daily time steps. Model calibration and residual analysis revealed shortcomings in geological representation and boundary condition realism. In response, the model was comprehensively updated. A 3D subsurface model based on a large set of borehole logs (developed via ArchPy and stochastic variogram-based simulations) provided a refined hydrostratigraphic framework. The domain was expanded using long-term average groundwater contours and regional geological maps, and information. The boundary conditions were upgraded to enable more flexible inflow estimation and a more realistic representation of surface water bodies interacting with the studied aquifer. The updated model was migrated to MODFLOW 6, using an unstructured vertex grid. A high-resolution mesh was used in the core domain and along key boundaries, while a coarser mesh was applied to the outer zones. This enabled detailed simulations to be performed at a reduced computational cost. Preliminary results show improved spatial groundwater level distribution and enhanced responsiveness to transient conditions. The general development of this model has provided an opportunity to make key methodological advances and gain early insights into groundwater flow dynamics. Future steps will focus on in-depth model evaluation and model application in water resource planning under climate stress. The numerical groundwater flow model introduced here will serve as a foundation to support robust decision-making for groundwater resource management for the city of Görlitz.