First, you cannot directly "connect" your Google Nest doorbell to your Nest Hub; you have to add or install each device in the same Google Nest "home/structure" in the Google Nest app or the Google Home app.
Second, whether you install your Google Nest doorbell in the Google Nest app or the Google Home app depends on your doorbell. The Google Nest Doorbell (Battery) and the 2nd Gen Google Nest Doorbell get installed in the Google Home app; the Google Nest Hello Doorbell (which has been around for years) gets installed in the Google Nest app (but then can be viewed in the Google Home app).
@AlaHassan I'm dropping by to ensure that everything is covered here. I hope that the information sheds some light on the issue. If you have other questions and concerns, feel free to let me know.
I appreciate the help, Brad and MplsCustomer.