Build web app functionality behind an Api

I have to prepare a specification to provide a road-map for developers to build an in-house project. The project will consist of a web application and a mobile application. The mobile app will be used to collect user feedback, typically the mobile app shall display a couple of questions for the user to answer.

Example questions show below;

1. Did you use shoe polish to clean your shoes ?
2. Did you watch the news at night ?

The data captured will be sent to an sql server.

The web app shall be used to publish the questions to the mobile app, the web app shall also be used to view reports. The web app shall have below features; 1. Publish surveys to the mobile app, this could be done by MQTT,AMQP or similar protocol. 2. View data in chart form 3. Manage devices, like enrolling new mobile devices e.t.c

What is needed This project will be spit and assigned to 3 teams, the backend team(Api team) , the frontEnd team and the Mobile developers team. The functionality of the backend is supposed to go into an Api, the front end should always talk to the backend for getting data, basically no business logic is allowed to go into the front end side of things. The front end will only write css/html/js for markup & presentation, the rest of the functionality should be consumed through an Api.

I have to write a detailed specification of how the project should be implemented, the back-end will be implemented in PHP with symphony. The front-end can be any JavaScript framework, the mobile app will be implemented in Android.

Can you should how i should model the back-End(Api) so that it contains all the functionality needed in the web app ? On top of that, building the functionality on top of an api a good strategy for this project? Should i go the monolithic way as having the front end and back-End attached together(this will make it difficult to have one developer working on the front end and another one on the back-End/api ) ?