Yes, that is what the extensions directory is for. It’s been awhile since I’ve created an extension. Basically your folder name is the name of the extension and you create files in directory paths within it that are used instead of the standard file. For class files in the Inc/Claz folder, there is a trick to only add the class code that is specific for your need so you don’t have to copy the entire class and thereby put general maintenance of all the class code on you. Otherwise, you might miss some needed changes that future updates provide.
Maybe if you can explain what you want to do, I can see what it takes to set it up.