How to go from theoretical Concept of Layers (Input, Hidden and Output) of AI Neural Network and Write in CODE

I am currently working on a project to create a working Ai Neural Network in C++, I have watched and read many articles on Layers concept but I still don't know from where to start with the layers things, like do I need a specific function or something like that, since we input x and compute to get y the output.

Everything seems quite vague about this.