Thanks for trying out the game and glad you like it.
I also didn't know Godot could do this until that time that I was just looking at my daily youtube recommendations to see WindowKill and was very facinated by the idea. So I went to do some research and found out the game was made with Godot as Godot 4.0+ had improved Window features that also allows Windows to not be embedded (Be separated from the main window)
Someone made a really good tutorial on Github so I decided to follow it.
I didn't make a lot of levels as this was my first time using Godot so I needed to learn a lot of stuff.