As best I can understand, K is proprietary and you need to have a business relationship with Arthur Whitney to get access to the latest. You can get a free trial for personal use here: https://kx.com/download-kdb/. There are some open source clones like: https://codeberg.org/ngn/k https://github.com/kevinlawler/kona https://github.com/JohnEarnest/ok , and inspired languages: http://t3x.org/klong/

I've also heard it has a really interesting system for creating UIs but cannot find anything about it

Here's the ngn/k opensource implementation I used for AOC last year https://codeberg.org/ngn/k