Having trouble comprehending OOP principles of accessing objects outside of class

I’m having a bit of trouble understanding how to access objects (object instantiated from other classes) from within a class. There’s so much spaghetti code to go through, right now I’m porting all timeline code in one project to different classes. What are the different ways of accessing objects or classes from within one class? Don’t get me wrong, I understand that you have to instantiate a class and then use its name plus the dot character to access any properties, but sometimes I get confused and would just like a simple explanation of the best way to go about accessing objects from within a class (and not just the main class).