I built GitHub's original mobile views. The team was myself and 1 engineer.
-
We built quick, simple prototypes in code to get feedback from everyone at the company via pull requests.
-
When something resonated, we would merge it, enabling it only for internal use, and continue to get feedback and iterate.
-
We ended up shipping the first version of GitHub for mobile devices in about 6 weeks.

At the time GitHub was a large, web-only Rails application and we had to figure out a way to dramatically reduce page size and avoid adding a significant amount of new CSS to the code base.
We ended up building separate web views rather than doing responsive views to avoid re-architecting existing CSS and sending huge page sizes over 3G mobile connections.

We didn't plan to invest a mobile team at the time, as GitHub was still small, so we built out a style guide and simple design system that allowed anyone to turn their feature into something that worked on mobile:
