Modify a Contrib Module

I am getting into developing Drupal modules and feel like I am kicking against the wall. After trying to find the answer on my own and failed I am deciding to join the Community and ask. I need some direction on modifying a current contrib module (project/physical). I would like to add override some of the functions in the module and add a couple of new options. I already have looked through the code and know what I need to change and the functions to override. I could just change the code, or copy the module and make my own, but those both seem wrong. Where I am stuck is how do I structure my module so that when enabled it extends or overrides the physical module with my code. I don’t want to recreate the wheel. I have already created a module using Drupal console.

Can someone direct me to a tutorial or give some advice on where to start or how to accomplish this? I’ve found code of how to extend or override the actual functions, but how do I structure my module to run on top of another module?

Thanks in advance!

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