Have you tried this learning path? https://academy.mendix.com/link/paths/33/Create-a-REST-API-(Micro-Learning)-
It should be enough to show you how to create a Published Rest service. The assumption is that you’re already familiar with APIs in general (Headers, Authentication, REST & XML/JSON) – that isn’t specific to Mendix. Postman should also be your best friend.
You’ll need to build an API for each Mendix app.
Consume a REST service: https://docs.mendix.com/howto/integration/consume-a-rest-service/
Or try one of these videos: https://www.google.com/search?tbm=vid&q=mendix+consume+rest