What are code mods
Code Mods are mods that alter the games code.
Note that code mods are only supported on PC and are incompatible for the mobile versions of the game.
They are a part of authorised game mods seen here.

Installing Code Mods
Most Code Mods would have a github.
Lets use Astro the Rabbit's Aero Trajectory Mod as Example

Here you can see there is a download button that downloads the latest version of that Mod but lets press on sources since some mods just give you a github page

Now here you should press releases
After that you should be taken to a page that looks like this

Clck on the .dll to download the mod
Now you need to navigate to "Spaceflight Simulator Game/Mods" and put the .dll there
If you done everything correctly this is what it should look like

Now open the game and it should put the .dll in a folder and it should also appear in the Mod Loader.
If it does then congrates you done it correctly.
Enjoy your mods!