Google Calendar help/backup

Hi, I have 20 years of calendar history which I have never backed up. I want to back this up. What’s the most efficient way of doing this?

Also, the calendar app on android/ios shows events up to 2 years or so in the past. I want to be able to see everything going back 20 years. How do I do this? Can I import my calendar data to an offline app perhaps?

Thanks