Designing assistive technologies for the benefit of a population of multi-disabled users living in specialized care homes involves the implementation of a user-centered approach (UCD). The purpose of this article is to demonstrate how the ecosystem also plays a key role in the codesign and appropriation phases of a augmentative and alternative communication (AAC), mainly in the choice and adjustment of interaction techniques through a case study. Firstly, we present related works on the codesign of assistive technologies. Then, we will describe the SoKeyTo platform used for the codesign by the human-computer interaction team. We will also explain the collaboration between the ecosystem and HCI team during tests (place of the switch and type of scanning). Usability criteria were used to determine these choices. Finally, we will illustrate our approach in the codesign and appropriation activities of layout and interaction modes of the AAC.