The “idea for an app” discussion has become a joke among many developers.
Pragmatically, the idea itself is a rounding error in the effort involved in building and selling and supporting an app, or in building a business to sell that app.
Identifying a profitable market or even a niche is the most important part of any product development effort. A profitable niche for even a bad idea is still a profitable niche.
But do know what is being sold. It might not be the app itself, even if that too is being sold.
Sometimes, the person with the idea is themselves the profitable market. Sellers of vanity apps and books exist.
Sometimes, building and selling the business itself is the goal. Investors are usually looking for a payout. That payout might not be the path that the business creator wants. A whole lot of the Silly Valley ethos—the quiet part—is about getting others to build a business for you, and then selling “their” business.
If you want some business things to think about, rummage through the Patio11 archives:
… https://www.kalzumeus.com/greatest-hits/
But to your question, there are myriad search hits for, for instance, “App Development Case Studies”.