An ICS calendar, also called an iCal link, is a web address that other calendar apps can read to see which dates are booked. Airbnb, VRBO, and Google each publish one for your listing.
In Airbnb, open Listings, pick the listing, then go to Calendar → Availability → Connect calendars → Export Calendar and copy the link. Airbnb gives one link per listing, so copy each listing separately.
In your VRBO owner dashboard, go to Calendar → Export Calendar and copy the iCal link.