An app is a working system. A system can be a daily text file of work notes. It's no code, only process. It may be informal, known only to the author. When they hire a worker to delegate to, they train the worker about the system. The worker may be proactive and document what they were taught. One day perhaps they will delegate the use of the system to another still.
When the system is formalized it becomes an App. Not yet a product or project, just an app. A documented system that meets the needs of the owner. As users are added to the system, new concerns arise. Merging notes, overwriting work, all that comes with concurrency and perhaps parallelism.
To address these new concerns, new project artifacts must be incorporated. Code bases, external dependencies. This is where the role of developer becomes meaningful, and the wisdom of software engineering is critical but perhaps elusive. Software engineering is software development over time.
Time brings change. Evolving duties and user profiles, external dependencies and more. Yet there is business to be done while these changes are incorporated into the system, apps and infrastructure. There may be formal service level agreements (SLAs) that must be met or a penalty paid. The development and operational processes become complex and have costs associated with them, and decision making must be managed and accountable.
This is a far journey from author managing a set of note files. Experts have traveled the road many times before and know what's coming. They leverage whatever tools are available ad know how to pop the hood and quickly assess situations and make small or large improvements. Whereas a new developer may be on their first journey and only have read the map, or not.
The level of developer required for an app depends on factors like cost of failure, time to market requirement and anticipated growth factors. Describe your custom app idea and we'll put together a plan to engineer it right.