Faced with such a problem. I linked my blender and PyQt6. We even managed to find an example of how to start the process without the main window of the blender hanging. However, when I started to complicate the design, namely! If I try to switch windows via QtWidgets.QStackedWidget(), then I can’t add the same switch. With grief in half, I found which part of the code is responsible for the part I need and entered it there

self.widget = QtWidgets.QStackedWidget()
self.widget.addWidget(self._widget(*self._args, **self._kwargs))
self.widget.show()

however, this did not affect access to the variable as I expected
I will attach the code that needs to be corrected

import os
import bpy
import sys

sys.path.append('/home/user/venv/lib/python3.12/site-packages')
import logging
from PyQt6 import QtWidgets, QtCore
from PyQt6.QtWidgets import QDialog, QStackedWidget, QApplication

logger = logging.getLogger('qtutils')


class QtWindowEventLoop(bpy.types.Operator):
    """Allows PyQt or PySide to run inside Blender"""

    bl_idname = 'screen.qt_event_loop'
    bl_label = 'Qt Event Loop'

    def __init__(self, widget, *args, **kwargs):
        self._widget = widget
        self._args = args
        self._kwargs = kwargs

    def modal(self, context, event):
        # bpy.context.window_manager
        wm = context.window_manager

        if not self.widget.isVisible():
            # if widget is closed
            logger.debug('finish modal operator')
            wm.event_timer_remove(self._timer)
            return {'FINISHED'}
        else:
            logger.debug('process the events for Qt window')
            self.event_loop.processEvents()
            self.app.sendPostedEvents(None, 0)

        return {'PASS_THROUGH'}

    def execute(self, context):
        logger.debug('execute operator')

        self.app = QtWidgets.QApplication.instance()
        
        # instance() gives the possibility to have multiple windows
        # and close it one by one
        if not self.app:
            # create the first instance
            self.app = QtWidgets.QApplication(sys.argv)

        if 'stylesheet' in self._kwargs:
            stylesheet = self._kwargs['stylesheet']
            self.set_stylesheet(self.app, stylesheet)        


        self.event_loop = QtCore.QEventLoop()
        self.widget = QtWidgets.QStackedWidget()
        self.widget.addWidget(self._widget(*self._args, **self._kwargs))
        self.widget.show()
        #self.widget = self._widget(*self._args, **self._kwargs)
        
        logger.debug(self.app)
        logger.debug(self.widget)

        # run modal
        wm = context.window_manager
        self._timer = wm.event_timer_add(1 / 120, window=context.window)
        context.window_manager.modal_handler_add(self)

        return {'RUNNING_MODAL'}

    def set_stylesheet(self, app, filepath):
        file_qss = QtCore.QFile(filepath)
        if file_qss.exists():
            file_qss.open(QtCore.QFile.ReadOnly)
            stylesheet = QtCore.QTextStream(file_qss).readAll()
            app.setStyleSheet(stylesheet)
            file_qss.close()


class Login_Form(QDialog):

    def __init__(self, parent=None):
        super(Login_Form, self).__init__(parent)
        self.setFixedSize(261, 250)
        self.setWindowTitle('login form')
        self.log = QtWidgets.QLineEdit()
        self.label_log = QtWidgets.QLabel("Login")
        self.passw = QtWidgets.QLineEdit()
        self.label_passw = QtWidgets.QLabel("Password")
        self.login_botton = QtWidgets.QPushButton(text="login")
        self.login_botton.clicked.connect(self.login)
        layout = QtWidgets.QGridLayout()
        layout.addWidget(self.log, 0, 1, 1, 1)
        layout.addWidget(self.login_botton, 5, 0, 1, 2)
        layout.addWidget(self.passw, 1, 1, 1, 1)
        layout.addWidget(self.label_passw, 1, 0, 1, 1)
        layout.addWidget(self.label_log, 0, 0, 1, 1)
        self.setLayout(layout)
        #self.show()

    def login(self):
        second_window = Second_window()
        widget.addWidget(second_window)
        widget.setCurrentIndex(widget.currentIndex()+1)


class Second_window(QDialog):
    def __init__(self, parent=None):
        super(Second_window, self).__init__(parent)

   # def show_second_window(self):
        #=====================================================================================
        self.setFixedSize(500, 500)
        self.setWindowTitle('main')
        layout = QtWidgets.QGridLayout()
        self.setLayout(layout)
        #self.show()


class RedmineWindowOperator(QtWindowEventLoop):
    bl_idname = 'screen_login.redmine'
    bl_label = 'Cam Controller'

    def __init__(self):
        super().__init__(Login_Form)


if __name__ == '__main__':
    bpy.utils.register_class(RedmineWindowOperator)
    bpy.ops.screen_login.redmine()  # load the tool upon initializing the addon?

and the code that works well if you run it from the console (but there’s nothing there to work with blender, it’s just a test field)

import os
import sys
from PyQt6 import QtWidgets, QtCore
from PyQt6.QtWidgets import QDialog, QStackedWidget, QApplication

class Login_Form(QDialog):

    def __init__(self, parent=None):
        super(Login_Form, self).__init__(parent)
        self.setFixedSize(261, 250)
        self.setWindowTitle('login form')
        self.log = QtWidgets.QLineEdit()
        self.label_log = QtWidgets.QLabel("Login")
        self.passw = QtWidgets.QLineEdit()
        self.label_passw = QtWidgets.QLabel("Password")
        self.login_botton = QtWidgets.QPushButton(text="login")
        self.login_botton.clicked.connect(self.login)
        layout = QtWidgets.QGridLayout()
        layout.addWidget(self.log, 0, 1, 1, 1)
        layout.addWidget(self.login_botton, 5, 0, 1, 2)
        layout.addWidget(self.passw, 1, 1, 1, 1)
        layout.addWidget(self.label_passw, 1, 0, 1, 1)
        layout.addWidget(self.label_log, 0, 0, 1, 1)
        self.setLayout(layout)


    def login(self):
        second_window = Second_window()
        widget.addWidget(second_window)
        widget.setCurrentIndex(widget.currentIndex()+1)


class Second_window(QDialog):
    def __init__(self, parent=None):
        super(Second_window, self).__init__(parent)

   # def show_second_window(self):
        #=====================================================================================
        self.setFixedSize(500, 500)
        self.setWindowTitle('main')
        layout = QtWidgets.QGridLayout()
        self.setLayout(layout)

app = QApplication(sys.argv)
mainwindow = Login_Form()
widget = QtWidgets.QStackedWidget()
widget.addWidget(mainwindow)
widget.show()
app.exec() 

I tried different approaches that either led to a crash, or simply did not work as the last option…