The Google Classroom calendar has the flexibility of being stand-alone within Classroom, or view-able with your other calendars within Google Calendar. Which you (and your students) want to use depends on how you want to use it. One of the big differences is that the only way you can add items to the calendar built into Google Classroom is by adding them as an assignment, etc within the classroom; whereas the calendar as viewed within Google Calendar is editable - just like your other Google Calendars.
To view your calendar within Classroom, click on the 3 horizontal bars on the upper left side
Select Calendar -
This will bring up a calendar that is built into the Classroom, which will display the assignments for the course (or all classes) -
To view the Classroom calendar within Google Calendar, look for the classroom name right within your calendars list -
