What stage of creating a mobile app do you consider to be the main one?

I think the most important stage is the testing of mobile applications. After all, application quality is the key to success, and this can only be guaranteed by testing mobile applications. You can do that here https://rubygarage.org/services/mobile-application-testing . The success of an app can be measured by the number of downloads and positive comments, as well as the speed of implementing new features and fixing bugs.

This is really very important. After all, you could make a mistake and the application simply won't work.

Your mobile app likely began as a simple concept. The majority of your effort is still ahead of you after the initial spark. The first thing you should do is look into the current app ecosystem to check if your concept has already been implemented. See these helpful tips. No matter how excellent the app idea, if you merely copy an already-existing product and don't consider innovating & fixing difficulties with your mobile app development solutions, your work will be for nothing.

I agree, testing is one of the most important steps in mobile app development, so there should be an appropriate approach. It is important to develop documentation, I mean test plan