It’s possible to mod with certain limitations. Main menu modding is more of a reskin as a result though it is possible to hide elements by throwing them off screen. Most mods have various main menu modifications you can investigate to see what’s possible. 

Theres also some interesting modifications of the faction selection menu you should look at like in STA3. 

There are no tutorials. The best option would be to take existing mods to see how they have done things. It's not overly complicated as it's mostly re-skinning though you can get some creative results that way.