The key to building scalable mobile applications, according to Scott Campagna of FIS, lies in creating a solid foundation for your application by leveraging common skillsets and frameworks, automating infrastructure, and implementing modular pipelines that enable fast and efficient feature updates. By adopting automation tools like Bitrise, the team was able to streamline their workflow, reduce manual errors, and improve the overall developer experience, ultimately leading to better app quality and reduced issues for end users.