Geometry Dash, a game developed by Robert Topala, is a community driven game. A community driven game is a type of game that mainly stays alive due to an active community.
These types of games usually have "sub-communities." Geometry Dash has these sub-communities and one of them specializes into reverse-engineering, hacking, and modding the game. This is the hacking community. These very smart people created whole frameworks to handle Geometry Dash modding, and boy do they have a lot of dedication to do such a thing.
Geometry Dash is a video game coded in C++ with a framework called Cocos2d-x.
Because of the way that the game is built and compiled, the game is a .exe file, meaning that it's raw binary and there is no way that a human can read and write to the file. This is what us programmers call a "standalone application."
Hackers found a "loophole" in the game. If they modify memory adresses from the app, they could change some functionalities of the game, like no-clip. This did not have many use cases though, as you cannot create or delete GUI or other game elements by memory editing.
However, in November 2014, the 1.9 update came out, and just like that, the game was now on computers! Now that the game was on computers, hackers and modders could inject dll's into the game using modified game dll's and external libraries. This let modders and hackers create in game UI and game functionalities even faster and easier. This is also how the now extremely popular "Mega Hack V7" was created.
This part is more on the technical side, so it is not needed to read this.
To make a mod, you need five fundamental components. A coding environment (IDE, text editor, etc.), a compiler (LLVM & MSVC++ only), Cocos Headers, MinHook, and GD.h. With these, you can do a basic setup with this article by Matcool.
The Geometry Dash community has come a long way, from a simple mobile game in 2013, to a heavily modifiable game with a rich and active community. I hope this article helped you understand how Geometry Dash hacking and modding works. Now you know how all these extensions and mods are made as well as the work that was put into making these mods!
Like I said in the "How mods are made" section, this repository explains in way more detail how mods work, hooking, etc.