app development with backend and api

I am looking for a freelancer to develop an app for both iOS and Android, with an OpenSteetMap and OpenElevation API. While I don't have a specific design or wireframe in place yet, I do have a general idea of what I want for the app. In particular, I would like it to have user accounts, allowing them to login and access different features of the app such as maps, and route planning. There will also be some numerical method calculations and simple algorithms.