If you're looking for a place to start, I would recommend checking out the Engineering Design Process. This is an industry standard for the process of design and development, and as a mobile app designer, is one you will become quite familiar with.
The Engineering Design Process: https://www.eie.org/overview/engineering-design-process
Best of luck!
A mobile app developer is a computer software engineer who designs, creates and programs applications for mobile devices, such as iOS, Android, Windows, etc. This is a job skill that's in high demand, and will continue to be in demand as technology adoption grows even more in the future.
A mobile developer is software engineer who develops and implement new functionality, UX design and different application in mobile device. This is a good skill as more people as many people prefer to use mobile than in a UI/laptop/desktop.