In UML, you make a project where you specify a high-level model of the project.
Functions belong to the implementation. It contains many technical details that are not relevant to your model, so you do not put them in the model.
You can create a list of class functions. A good selection of their names helps to understand what they should do. For some of them, you can provide comments in which you explain how you are going to implement them.
Of course, you can imagine some high-level interactions in sequence diagrams.
Oleg Pavliv
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