As per Google Support, here are the basic steps. You can use either an ICS or a CSV file.
Open Google Calendar.
In the top right, click the Settings Gear icon:
Click Import & Export:
Click Select file from your computer and select the file which has the data you wish to upload into your calendar. The file should end in ".ics" or ".csv."
Choose which calendar to add the imported events to.
By default, events are imported into your primary calendar.
Click Import.
If you have a .zip file, find it on your computer and open it. You'll find .ics files for each of your calendars. Take the individual files out of the .zip file, and import each .ics file individually.
If you import repeat events from a .csv file, they might not show up that way. They'll be on your calendar as a series of one-time events.
For more information, check out this Google Support article: https://support.google.com/calendar/answer/37118?hl=en&co=GENIE.Platform%3DDesktop