Hi
Without knowing which device and app you have its hard to be specific, but, in most calendar apps there is a setting to add calendars to those displayed. The default is usually just a local calendar ("My Calendar" on samsung phones) stored on the device and not synced anywhere.
eg On most samsung devices, open the calendar app, press the calendar button at the top. Select "add account" and select your google account (having previously enabled the calendar sync in settings). You can add as many as you like - I have calendars from 4 different google accounts displayed in the calendar app on my phone.
Roger