What standards or procedures to follow for medical mobile app?

Answer:

Design and development of medical mobile apps should follow the local regulatory requirements in the country where you will be producing these mobile apps, and should also follow some guidance from the ISO 13485 standard. You can look at clause 7.1 Product Realization and 7.3 Design and Development for some basic guidance on the documentation. As for design specification of the UI/UX of these mobile apps, there are no standards nor other requirements, so you have to tailor them towards the intended use of the mobile apps.

For more information, please read article:
How to manage design and development of medical devices according to ISO 13485:2016
https://advisera.com/13485academy/blog/2017/08/24/how-to-manage-design-and-development-of-medical-devices-according-to-iso-134852016/