A lot of app building developers design, code, and modify mobile applications, from layout to function, according to clients' specifications. They use programming languages such as Java, Swift, or Python, and work with databases and APIs. They focus on creating user-friendly interfaces and high-performance apps, while ensuring the software's compatibility with various devices and operating systems. They also test, debug, and improve the apps to enhance their performance and security. Additionally, they collaborate with other team members, such as UI/UX designers and product managers, to create an app that meets the user's needs and the market trends.