An outside script to load layers in QGIS

Question

I have been piecing code together from a number of posts to python outside of QGIS that will open a QGIS instance and then load specific data I feed it. I am not to familiar with using the QGIS API commands, so I am a bit lost. I assume that I am not properly pointing to the instance that is being open. Seems like there are a lot of questions revolving around this topic, but I haven't been able to find something up to date (QGIS 3.16) or that uses scripting outside of the QGIS shell.

Any suggestions?

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

app_path = "C:/Program Files/QGIS 3.16/bin/qgis-ltr-bin.exe"

def open_app(application_path):
    subprocess.call(application_path)
    
def open_layers():
    qgishome = "C:/Program Files/QGIS 3.16/apps/qgis-ltr/"
    app = QgsApplication([], True)
    QgsApplication.setPrefixPath(qgishome, True)
    QgsApplication.initQgis()
    
    vlayer = QgsVectorLayer("D:/my_shapefile.shp", 'layer', 'ogr')

    ### EDITS ###
    ex = vlayer.extent()
    canvas = QgsMapCanvas()
    canvas.setExtent(ex)
    ### EDITS ###

    QgsProject.instance().addMapLayer(vlayer)

def main():
    open_app(app_path)
    open_layers()

if __name__ == "__main__":
    main()

Class Edits

With the advice from @J. Monticolo I created a class and defined self.canvas, but the same problem occurs. A small window appears and disappears as if the app loads and exits. I'll admit, I have not worked with classes much so I am a bit rusty.

class Test():
    def __init__(self):
        pass
        
    
    def open_layers(self):
        qgishome = "C:/Program Files/QGIS 3.16/apps/qgis-ltr/"
        app = QgsApplication([], True)
        QgsApplication.setPrefixPath(qgishome, True)
        QgsApplication.initQgis()
        
        vlayer = QgsVectorLayer("D:/my_shapefile.shp", 'layer', 'ogr')
        self.canvas = QgsMapCanvas()
        ex = vlayer.extent()
        self.canvas = QgsMapCanvas()
        self.canvas.setExtent(ex)

        QgsProject.instance().addMapLayer(vlayer)
        self.canvas.show()
    
def main():
    app = Test()
    app.open_layers()