Joint In is a pioneering modular system engineered to activate idle urban land, achieving rapid spatial transformation through high-precision standardized geometric parts and rPET recycled plastic. By seamlessly integrating a dedicated online configuration platform, the system returns land-use autonomy directly to the local community, fostering a bottom-up approach to urban planning. This collaborative framework empowers users to deploy circular, socially resilient temporary public spaces that adapt to shifting city needs, effectively bridging the gap between vacant neglect and community engagement.