You can swap out kwin for any other window manager if you want, say I3, sway, or DWM.

No clue if that's what they're doing, it looks manual to me with the odd spacing, and uneven windows, but here's a tutorial:
https://userbase.kde.org/Tutorials/Using_Other_Window_Managers_with_Plasma

I'd look for something distro specific though to be fair

Incase OP is reading, absolutely beautiful work. Very well done