Creating a structure for your app, however, can take a lot of time and energy, unless of course, you have the right tool in place. That is why many mobile developers start out with a mobile app wireframing or mockup tool. Mobile app wireframes can not only help save valuable time (and money!) but also help developers focus on the user experience.
https://mopinion.com/mobile-app-wireframing-mockup-tools/
Mobile first design is not a new concept.
It has been around for more than a decade and it’s used to refer to the strategy of designing and developing digital experiences for mobile devices at the very start of a project; rather than the traditional approach of first designing for desktop and consequently for mobile.
However, the term is possibly more relevant in the business world today than ever before.
https://www.marketingtechnews.net/news/2018/feb/15/prioritising-mobile-first-design/