Making accessible apps means developing products that are compatible with existing ATs available for each platform.
In most cases, app developers create their products without having accessibility in mind. This means that some of the information that the OS sends to ATs running in the system will not be available when the third-party app is in the foreground. As a result, this gives inaccessible apps. This lack of accessibility can be partial or complete, depending on what's missing from the development side.
Developing apps that are accessible through ATs and expose all the needed information for that is not difficult. Best practices on accessibility development allow engineers to create accessible apps with minimal extra effort.
Also, design of interfaces can be done following some best practices which will make it accessible for all users, no matter of what extra ATs they need in order to interact with the mobile device.