For Hotel bookings, many (all?) Expedia subsidiaries seems to be just a different UI for the same offer with slightly different prices since they target different users (premium vs budget etc). All bookings are made under the name Expedia.

In case of Priceline Group, it depends on the market. E.g. in China Agoda seems to be just another front-end for booking.com. All bookings are actually made through booking.com. But e.g. in Malaysia Agoda lists some Hotels which I could not find on Booking.com.

The Flight search engines seem to have major differences, e.g. in Southeast Asia Momondo lists a lot more local agents and more airline websites directly than Kayak.