[nostrademons]

Complements are essentially the "ecosystem" around your product. In a traditional microeconomics text, they're the goods that are commonly purchased alongside your product. So PCs, monitors, keyboards, and mice are all complements.

Re-reading my comment, I see some of the confusion. If you think of each of these companies as a reseller of books/websites/computers/rides, then those goods are not their complements; they're their suppliers. If you think of them as a marketplace that provides a service to find books/websites/[okay Apple makes no sense here]/rides and then takes a cut for their value-add, then they are complements: the driver and the service to find the driver are separate goods that must be booked together. There are substantial legal battles around which is a better model for the tech industry: Elizabeth Warren's campaign to prevent tech companies from also owning services that compete on their platform assumes they are marketplaces, the employee/1099 classification lawsuit assumes that they are resellers, the Australian law to force Google to pay publishers assumes they are resellers.

I could probably have picked clearer examples, eg. cheap TVs and floppy disks really are complements for 1975 Apple but chips are suppliers, while Amazon's own business plan implies that it's a retailer rather than a marketplace. Google is still a pretty good example, though: Google does not sell websites (or didn't in the 1990s, at least), but the need for a search engine arose out of there being many websites around.