This one sounds hard, but it isn't.
1. Copy this address:
2. Go to Settings on your phone.
3. This next step changes depending on the version of iOS you are running, but in iOS 13 it is Passwords & Accounts. (In an older version look for Accounts it used to be Mail, Contacts, Accounts.) Open that link.
4. Click on Add Account
5. Click on Other
6. Click on Add Subscribed Calendar
7. Paste the address you copied in step 1.
8. Click on Save.
That should be it. The calendar is now in your iPhone Calendar. If you have problems and would like my help, just ask.