Inspiration for this answer was found in @J.Monticolo's comment

sorry @taras, I missread. So, for me, put the startup.py file into a your_plugin_name_or_whatever.py, do a function called def launch_code and put the startup.py "code that runs the program" part into it, and then, in you plugin generated by pb_tool, put a call to launch_code in the def run, function called by plugin menu button click.

and in this article : Customizing QGIS with Python | 14.1 A Minimal Plugin.

This steps helped me to achieve the aim

Step 1. Put the content of the startup.py file inside of one function e.g. launch_code() in a new Python file

main.py :

def launch_code():
    imports

        import sys
        from PyQt5.QtWidgets import *
        from PyQt5.QtCore import *
        from PyQt5.QtGui import *
        from qgis.core import *
        from qgis.utils import *
        from qgspasswordlineedit import QgsPasswordLineEdit

    ----------------------------
    configuration files
    ----------------------------
    functions
    ----------------------------
    layouts / pages

        ###Layouts

        #Page1
        class IntroPage(QWizardPage):

            def __init__(self, parent=None): 
                super(IntroPage, self).__init__(parent)
                ...
        #Page2
        class LayerPage(QWizardPage):

             def __init__(self, parent=None): 
                super(LayerPage, self).__init__(parent)
                ...

        #page 3
        class AuthPage(QWizardPage):
            
            def __init__(self, parent=None): 
                super(AuthPage, self).__init__(parent)
                ...

        #QWizard Class
        class Window(QWizard): 

            def __init__(self):
                super(Window, self).__init__()
                #adds Pages to Wizard
                self.IntroPage = IntroPage()
                self.LayerPage = LayerPage()
                self.AuthPage = AuthPage()
                self.addPage(self.IntroPage)
                self.addPage(self.LayerPage)
                self.addPage(self.AuthPage)
                #Button check if Oracle or Postgre Data selected
                self.button(QWizard.NextButton).clicked.connect(lambda x:OraclePostgreCheck(self.AuthPage.frame_2, self.AuthPage.frame_4))
                #Readjust GUI size at buttonclick
                self.button(QWizard.NextButton).clicked.connect(self.adjustSize)
                self.button(QWizard.BackButton).clicked.connect(self.adjustSize)

    ----------------------------
    code that runs the program

        ### 
        #Programm starts
        
        #loadConfigCSV(URI,";")
        loadPaketConfigJSON(PURI)
        loadConfigJSON(URI)
        kategoriesJSON()
        #loadConfigtoQGIS() #load config for export during development
        
        
        #GUI
        window = Window()
        window.setWizardStyle(1)
        window.show()
        
        #Programm ends
        ###

Step 2. Create two Python files qgsdata.py and __init__.py that will bring this Plugin into action

qgsdata.py :

import os
import inspect

from PyQt5.QtGui import *
from PyQt5.QtWidgets import *

from qgis.core import *
from qgis.utils import *

from .main import launch_code

cmd_folder = os.path.split(inspect.getfile(inspect.currentframe()))[0]

class QgsDataPlugin:
    def __init__(self, iface):
        self.iface = iface

    def initGui(self):
        icon = os.path.join(os.path.join(cmd_folder, 'icon.png'))
        self.action = QAction(QIcon(icon), 'QgsData', self.iface.mainWindow())
        self.action.triggered.connect(self.run)
        self.iface.addPluginToMenu('&My Plugins', self.action)
        self.iface.addToolBarIcon(self.action)

    def unload(self):
        self.iface.removeToolBarIcon(self.action)
        self.iface.removePluginMenu('&My Plugins', self.action)
        del self.action

    def run(self):
        launch_code()

__init__.py :

# noinspection PyPep8Naming
def classFactory(iface):  # pylint: disable=invalid-name
    """
    Load QgsData class from file QgsData.
    :param iface: A QGIS interface instance.
    :type iface: QgsInterface
    """
    from .qgsdata import QgsDataPlugin
    return DataPlugin(iface)

Step 3. Create corresponding metadata.txt and pb_tool.cfg

Step 4. Run the pbt deploy command in the CMD and get a working QGIS Plugin under C:\Users\taras\AppData\Roaming\QGIS\QGIS3\profiles\default\python\plugins\QgsDataPlugin