Use a specialist; look at their experience. The application scope is not the only deciding factor. Often, customers select a contractor based on their portfolio. They might think, "these guys have already made an app for one restaurant, which means they'll do well making our app." But professionals on the development side know that similar previous work isn't necessarily proper for you.
In other words, apps created from the same components and controls will have similar features. The chat functionality in a dating app may suit you better than the other applications developed for food delivery services.
If your RFP is detailed enough, ask for estimated timelines and pricing. Customers often ask for an app but are unclear about what they truly want. They may try to compare the features and prices of different offerings before settling on a final decision. When a potential customer asks you, "How much does it cost to build a house?" imagine how we feel.
To ensure that your product is quantified according to accurate particulars, provide a detailed description of your services in an RFP (documentation after Feasibility Study). This way, the proposals from interested vendors should be more accurate and can compare competitors without worrying about contractors misunderstanding specifications.
Take on a trial contract with an agency or developer that will divide the work into specific agreements. If you need to develop an app from scratch, you'll need to first deal with business analysis and UI/UX. Isolate these tasks into individual contracts and use them to test your potential vendor.