You can click on the blue arrows here to download tarballs https://opensource.apple.com/release/macos-10126.html

Kudos! I'd love to see the game logic.

The game engine itself is a pretty much off the shelf version of Sjeng: https://www.sjeng.org/indexold.html

That's interesting, given Chess has a near identical UI to the old NeXTSTEP Chess.app, which predates Sjeng!

The initial board perspective is similar, but otherwise, the two apps are quite different (One obvious difference is that NeXTStep Chess.app used a bitmapped board with a fixed perspective).

For comparison, the source of the older Chess is also available, in https://opensource.apple.com/release/mac-os-x-1015.html though some parts might no longer compile.