The modular phone is becoming more of a reality with Project Ara and Google have now shown developers just what is required to create swappable parts for the modular device.
As part of this, Google has posted the Module Developers Kit (MDK) which contains the information required for developers to get started.
Essentially, Google, and only Google at this moment in time, create the skeleton of the devices and the slots are then populated by third parties. This creates a lot of combinations and form factors of devices for Project Ara, some of which are detailed in the guide which breaks the devices into three sizes.
Google is also planning on putting together an online marketplace to offer guidance and the tools for customers to create their own modular phone, and whilst the developer kit is still very immature, it shows Project Ara is gaining some traction.