Building an app for electric bikes as a solo developer

I got to interview a developer from an Estonian startup called Ampler Bikes. He's a full-stack developer (specializing on mobile apps) who was given a chance to build an app for electric bikes(whaaat? how awesome is that???).

What makes the project special(besides the purpose of the app) is that the author was the only developer, thus it was he who had to choose the tech stack, make architectural decisions, etc.