1. First of all, in the foldable phone, they have two parts like unfolded it means primary and folded it means that cover screen. Then you need to understand all the features of mobiles and know-how to develop an application and how to give a smart way to develop it. On the other side, the foldable mobile device is working as a smartphone it is operated normally.
In these two parts, mobile has looked different in both parts in the unfolded part, it will look like a tablet, and in folded part, it will look like a smartphone.
2. In the next step make your app resizable. In these mobile it will work on both screens so, you need to know that make app foldable with the multi-frame environment. Make all functions of the app as per the mobile devices.
3. After complete, the second step needs to compatibility device with the checking of screen ratio. Test your app on multiple screens like with foldable screen, long or thin screen, minimum touch screen these all types of screens ratio is important for the test it.
4. In these steps we need multiple apps running side by side, when the phone is expanded into large size running at the same working app is a need. When mobile is in multi-resume means that the app stays in a resumed state. If the mobile is on the multi-mode same thing is a need.
5. After all testing like multi-display, screen ratio, multi-window, multi resume then foldable testing comes to testing. To make sure if a mobile app is ready for release, then tested it for a foldable device. As a developer, how your app working like configuration, multi-window & multi-resume is ready with the app, screen ratio, resizing, etc. These things are the responsibility of the developer in developing time and after developing time also.