Hi Maurizio, we talked about this via a separate support channel but I also wanted to post the answer here in case anyone else ever wanted to know.
We cannot guarantee that AAB, APK, or iOS project download files can be modified in any way outside of the Thunkable platform. Like yourself, we have several freelancers and agencies that build apps for clients and charge a fee for this service, but typically they share the Thunkable project file (so the client can manage the project in their own Thunkable account) or publish on the client’s behalf and provide ongoing maintenance services as needed.