With iCloud it supports a lot more Calendars than just the default Apple ones. Here are the steps to add more Calendars allowing you to import & sync more plans.
Adding New Calendars:
1. Go to Settings > Calendar > Accounts > Add Account.
2. Once added make sure you enable "Calendar"
Adjusting Enabled Calendars
1. Open the Calendar app
2. Tap Calendars at the bottom
3. Enable/Disable any calendars
This allows you to add popular calendars like: Google Calendar, Proton, Outlook