Fundamental OOP in Tkinter

I'm relatively new to OOPs and thus know it theoretically. I need a project to be implemented in Tkinter. So, I was going through it. I was just playing around with the most basic code like below

# https://python-textbok.readthedocs.io/en/1.0/Introduction_to_GUI_Programming.html
import tkinter as tk

class MainApplication(tk.Frame):
    def __init__(self, parent, *args, **kwargs):
        tk.Frame.__init__(self, parent, *args, **kwargs)
        self.parent = parent


root = tk.Tk()
app = MainApplication(root)
app.mainloop()

Here, I can say confidently that MainApplication inherits tk.Frame. So, the root is a TK object and it is passed in MainApplication constructor which assigns it as the parent of the MainApplication object.

So, when we call the app.mainloop(), the MainApplication object is called which in turn can invoke the root somehow because we have the root as a parent of the app object.

Correct me if I understood it wrong.

Now, as I browsed and another popular method people are using is

# https://docs.python.org/2/library/tkinter.html#a-simple-hello-world-program
import tkinter as tk

class MainApplication(tk.Frame):
    def __init__(self, parent, *args, **kwargs):
        tk.Frame.__init__(self, parent, *args, **kwargs)
        self.parent = parent


root = tk.Tk()
MainApplication(root)
root.mainloop()

Here also, as I can understand the MainApplication is invoked and root becomes the parent of it. But how does that link the MainApplication object with the root object, so that when the root.mainloop() is called, it can refer back to MainApplication object?

Any help is appreciated. Thanks in advance.