How to call functions from seperate module, without them being under the same structure?

I want to create a family of assistants that its functions or classes can be called from central window (user form) and run seperately as well. Therefore, for me to be able to create classes and structs in my own central library is necessary. In documentations, I have founds references to call functions only when they are under the same Assistant structure, but how do I do it from other assistant structure?