Apple - How are these Chinese spam iCloud Calendar/Reminders invitations sent to me?

It's simply that anyone can send an email to you. If spam rules don't block it, Apple software (with default settings) processes the invitation before you can take any action.

To prevent this automatic processing, try this.

Log in to iCloud on the web, open your calendar and go to ⚙ > Preferences (in the lower left corner). Under Advanced tab you’ll find an option to receive calendar invitations by e-mail instead of directly to your calendar.