Google Calendar does not show calendars by DAVx⁵
DAVx⁵ will sync data to the Android Calendar storage from where all other apps can read/write data to it.
Google Calendar does by default not show DAVx5 synced calendars. To enable it open the side drawer and go to Settings → Manage Accounts. In the list of Non-Google-Accounts activate the DAVx5 account(s). After that you'll be able to select various calendars from the side menu so that they are displayed in the calendar app.