If you are just now downloading the Google Home app for the first time, then are you possibly working with a Google Nest account that has never been migrated to a Google Account. (We migrated in 2020.). If you have migrated, then when you log into the Google Home app with the same gmail username and password you are using with the Google Nest app, then you should see the same "home" you set up originally in the Google Nest app, and your Google Home app should show a plus sign in the upper left corner, as shown in this Android screen capture. (The iPhone version is similar.)
If you have NOT migrated your Google Nest account to a Google account, you will need to do that in order to set up any of the newer Google Nest cameras and doorbells released starting in 2021, since they work ONLY in the Google Home app. You can find migration instructions here:
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