Seperate Web Framework From Main Application

I have a question regarding software design/architecture. I want to build an application, which provides a REST API and runs several algorithms in the background. My main aim is to seperate these two functions strictly. Ideally it would not matter for anyone developing the algorithms which framework is currently being used. The web framework would be resposible for providing the API and making calls to any service/database needed for the main application.

My first guess to accomplish such an architecture would be to have an "internal API" between the web framework and main application. I am at a loss though as to how to design such an API. The implementation would be in Java, but I think the first aim of the architecture should be to give a structure that it could be implemented in any OOP language.

Since I am unexperienced in designing software I want to kindly ask if such an architecture makes sense and if anyone could point me to resources, which describe something similar. So far I could not really find what I want. Maybe I am simply missing the right keywords to describe my problem.

Thank you for your help.