Custom development in a Composer world

Now that we are all used to using Composer to require and update any site, and automation to build sites on various platforms, where shall we put our custom work on the site? There are some who feel there should be a custom repository, with code which can be pulled in by the composer, while others feel the custom work is part of the site. And what about components that can be shared between projects? In this discussion, we will look at development use cases for all of the above and discuss ways projects can share, reuse and work together. We will also discuss what Composer Installers are and how they work to ensure your code is added in the proper place.