Designing complex systems like aircraft is increasingly challenging due to the growing number of systems and stakeholders involved. These systems comprise numerous interconnected subsystems developed by different teams, requiring designs that meet high-level requirements while managing constraints to ensure safety, performance, and compliance. This paper presents findings from various projects focused originally on enhancing simulation capabilities for an aircraft systems developer and integrator. The project's scope then expanded to improving collaborative design practices for greater efficiency and agility. The goal is also to better align systems engineering and simulation activities, ensuring continuity and improving overall effectiveness. The developed processes and tools presented in this paper address real-world complexity and are designed for industrial use, including robust configuration management and data protection. Built on open standards (SysML, Modelica, FMI etc.), they try to integrate smoothly into industrial platforms. The current prototype reflects years of adaptation to constraints and evolving practices at Dassault Aviation and among its design partners.