TL;DR
Apple Maps has now started mapping indoor layouts of 30 airports across the globe with primary focus on airports in the continental U.S. such as JFK, LaGuardia, LAX as well as international airports in Geneva, Amsterdam, Hong Kong International, two in London, Berlin and a few more in Canada.

Looking at the level of detail, it seems like Apple has been working on this for quite some time as only close cooperation with the airports, in-person surveyors, and people recording all the data in-person without relying on the street mapping cars Apple and Google love to overuse is what gets you detail like this.
If an airport is present in the list, it will now have the “Look Inside” option on Apple maps. Clicking on that brings up a bird’s-eye view of the inside layout of the terminal. In the case of most airports, that’s about it. Unless you’re giving a street view-esque experience, a lot of the airport terminals aren’t much to look at.

Features of Indoor Apple Maps
On the first grid view, each gate is highlighted in yellow and key points of interest like baggage, check-in counters, airport lounges, and security populate the map.
Zooming in further reveals more POIs in detail, namely restrooms, restaurants, shops and more. Tapping on a restaurant brings up a tab with information about the place just like a display panel outside the airport provides. The terminal name, floor number, phone, open hours and reviews from a third-party review platform (Yelp) are all displayed on this data tab.
The upper-right corner number toggles between terminal levels. Swiping up with two fingers brings up a 3D view of the space including depth which gives wandering people a clear picture around the terminal.
The system amazingly has an accuracy within a few meters of your location inside the terminals. Apple achieves this using Wi-Fi access points throughout the terminal for triangulation.

The bottom line, the ability to see which gate your flight is leaving from and finding out the exact carousel your luggage is on directly through Apple maps could prove to be extremely useful.
You can find the full list of airports carrying the new feature here on Apple’s site.