SimpleRockets 2 has a set of stock modtools that greatly simplifies the creation of mods for non-mobile platforms. The game allows for simpler modifications that do work on mobile (like adding packs of prebuilt subassemblies, allowing them to be shown in the parts list with custom icons or adding custom themes to change the colors of the GUI), but this post is going to be dedicated to the computer-only mods.
To get you started Aram has made a short-ish video tutorial explaining the basics, and if you like a more in-depth text version, Nathan has made a suite of tutorials on the basics, from installing Unity to adding custom elements to the interface:
He's also released a couple of script examples for you to try out:
There are also some modding community tutorials made by MarioG:
If you want to be part of the modding community, they've gathered in a Discord Server under the name of Complex Rockets. If you want to give modding a try and you want to ask questions directly to the existing modders and to some of the developers of the game you are welcome! We have a small modding community, there's a huge potential waiting to be uncovered!