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: