haven't used yet, what code does it use
The engine, and your games, are written using modern Object Pascal. See https://castle-engine.io/features , https://castle-engine.io/modern_pascal .
Note that we plan to release new engine version, 7.0-alpha.2, soon (in a week).
It will be available here (on itch.io) as well as on our webpage https://castle-engine.io/ . For today, if you want to try out the engine, I would recommend checking out our snapshot version, which is the main download on our https://castle-engine.io/ .