Drawing on the sociocultural framework of Communities of Practice, this study analizes the co-design and implementation of ten learning scenarios within the DataScEd4CiEn project. The research investigates how transdisciplinary learning scenarios function as macro-level boundaray objects that facilitate the reification of practice among researchers, teacher-educators, and in-service teachers from Cyprus, Germany, Greece, Ireland and Spain. Using a mixed-methods approach, the study traces how micro-level boundary objects—such as social justice problems, datasets, design templates, and inquiry questions—support collaboration across professional boundaries and curriculum subject areas. The analysis follows the trajectory of scenario integration across four phases: emergence, exploration, immersion, and consolidation. During the emergence and exploration phases, the study examines how these boundary objects were employed by researchers and teacher-educators, acting as brokers supporting teachers in navigating the disciplinary "silo effect" and coordinating the co-design of scenarios aimed at advancing data science education within STEAM for civic engagement. In the immersion phase, the focus shifted to classroom enactment, analyzing how in-service teachers and students use these boundary objects to connect complex social justice issues with curricular learning goals and to transition from descriptive to exploratory data practices while fostering civic engagement. Finally, the consolidation phase, informed by teacher reflection sheets and focus group discussions, examines how these practices became integrated into the teachers' shared repertoire. The findings suggest that learning scenarios grounded in meaningful social justice contexts can function as generative mediating structures in in-service teacher education, supporting the integration of data science within STEAM while promoting collaborative inquiry and civic engagement.