Switching between multiple pages

[LT-K101]

Hi guys, I saw this code online and I'm confused what the pages.ui line is? Can anyone please explain that line to me please? Because the code is switching between just two windows and the pages.ui seem to be a third ui window.Thanks in advance.

from PyQt5 import uic
from PyQt5.QtWidgets import QApplication, QMainWindow

from first import First
from second import Second

class MainWindow(QMainWindow):
    def __init__(self):
        super().__init__()
        uic.loadUi('pages.ui', self)
        self.first = First()
        self.stackedWidget.addWidget(self.first)
        self.first.change_btn.clicked.connect(self.go_to_second)
        self.second = Second()
        self.stackedWidget.addWidget(self.second)
        self.second.change_btn.clicked.connect(self.go_to_first)

    def go_to_first(self):
        self.stackedWidget.setCurrentIndex(0)

    def go_to_second(self):
        self.stackedWidget.setCurrentIndex(1)


if __name__ == '__main__':
    app = QApplication([])
    window = MainWindow()
    window.show()
    app.exec_()

first.py

from PyQt5 import uic
from PyQt5.QtWidgets import QWidget

class First(QWidget):
    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)
        uic.loadUi('first.ui', self

second.py

from PyQt5 import uic
from PyQt5.QtWidgets import QWidget

class Second(QWidget):
    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)
        uic.loadUi('second.ui', self

[JonB]

@LT-K101
The pages.ui is the designer file just for the QStackedWidget (self.stackedWidget), e.g. see https://www.tutorialspoint.com/pyqt/pyqt_qstackedwidget.htm? That is the "container" widget which holds references to the individual page widgets and allows switching between which (single) one of them is shown at any time.

[LT-K101]

@JonB Thanks a lot for your response, Its clear now.