You can create a website or application for your business or personal use with the assistance of a web development agency. While each agency has a different approach to building websites, most custom web development agency use a similar set of steps.
In general, I would recommend adhering to universal principles when selecting quality companies. However, until you try most of the known alteratives, you will not be able to choose the optimal one for myself - this is how I often do, including for https://apps.apple.com/us/app/cleanup-app-phone-cleaner/id1541639300. But to organize a comprehensive process requires a better system.
Depends on your needs. Just because some people develop websites, others create marketplaces.
Personally, I prefer the second variant which is building marketplace platform. I think it is profitable and zeroed risking variant.