What the Ara team and its outside collaborators have created is a platform that supports three sizes of phone: mini (rather basic), medium (mainstream) and jumbo (an oversized, phablet-style variant). The size of each is determined by its endoskeleton, or endo for short — the one component of an Ara phone that will be Google-branded, as opposed to being devised by a third-party company.
The endo is an aluminum frame that contains a bit of networking circuitry so the modules can talk to each other, a tiny back-up battery and not much else. Everything from the screen to the processor to the battery is provided in the form of a module — the medium-sized endo has space for ten of them — which you slide into place to form a phone. In the first prototype, the modules use retractable pins to connect to the endo’s network; later this year, Google plans to replace that approach with more space-efficient capacitive connections.
Like the expansion slots on a desktop PC’s motherboard, each compartment on the endo is designed to handle any module of the correct size, regardless of its function. Though basic technical issues are sometimes a factor — an antenna can’t just go anywhere on a phone’s body, for instance — the general idea is to design the phone so that you can swap modules in and out at will.