The QMainWindow Class

1. Menu Bar: this is the menu bar. This is where you will be able to create your File, Edit, View, Help, etc...
2. Toolbars: the toolbars. In a text editor, for example, we have icons to create a new file, to save, etc... 
3. Dock Widgets: more complex and more rarely used, these docks are containers that are placed around the main window. They can contain tools, for example the different types of brushes that one can use when making drawing software. 
4. Central Widget: this is the heart of the window, where there will be the actual content. 
5. Status Bar: this is the status bar. It usually displays the status of the program: Ready, Recording in progress etc...

3 - First QMainWindow

import sys
from PyQt5.QtWidgets import QWidget, QMainWindow, QApplication

class My_MainWindow(QWidget):
    
    def setupUi(self, winObject):
        winObject.setObjectName("My_Main_Window")
        winObject.resize(500, 300)
        winObject.setWindowTitle("This is my first MainWindow ! ")
        winObject.setGeometry(100 , 100 , 500 , 300)
        
if __name__ == "__main__":
     
    app = QApplication(sys.argv)
    MainWindow = QMainWindow()
    My_UI = My_MainWindow()
    My_UI.setupUi(MainWindow)
    MainWindow.show()
    sys.exit(app.exec_())