You are right. In fact Google flights is regularly missing some airlines. Also the flights I found are often great on Google but then not available when actually trying to book, making the site quite a bit useless.

What sets it apart is the UI and fast search. But I guess with fast searching (caching) comes inaccurate results. It’s a thin line.

I'm always curious about where these sites source their prices from. Do the airlines provide a backend with real-time prices? Are they scraped from the frontend? Do these sites sell at a loss sometimes in "guaranteeing" a lower price?

I've known of a service that actually provides APIs. I cannot remember what it is called but such services exist for hotels and holidays too.

Iirc I stumbled upon one of the flight tracking services by pure chance.

Having said that, I feel like Skyscanner at the very least has their own systems in place. There have been times where I've chosen a ticket, have gone through to the website that sells the ticket at the price, and just before it goes through, Skyscanner validates that the price still exists, and then bails saying the price has increased or decreased. Usually it's not by a lot, but it has happened.