In sim games you usually get the game and then get a vehicle you want to specialize in. You may put hundreds or thousands of hours into that vehicle and you know the ins and outs.

Mod development wise you may have to model the vehicle from the ground up. The vehicle has to have functioning gauges, be tuned to handle and behave like the vehicle in real life, fail like the real thing and basically BE the real thing. Take DCS for example. The game itself is actually free but the modules are paid. The detailed modules can go for around $70 or so dollars like the F16. You can use the real life flight manual for the F16 to learn how to use it in game. Train sims are similar. If you are real particular about a specific train you can find the module for it and expect probably pretty damn good detail down to nameplates and specifications

You arent going to get that sort of involvement in a game like Skyrim, Fallout, or Cyberpunk. You could maybe go down to that level of detail but that level of detail may not be able to be appreciated in a game that doesent simulate things like aerodynamics or how tire grip changes depending on temperature.

Mods are and should be a passion project. The moment you implement mods as income stream people and companies are going to exploit the shit out of it. See: Roblox, Skyrim, etc

Sim games are sorta their own beast when it comes to mods.

Whats confusing about it? People took something that was going bad and made it good again. Thats the benefit of open source. Even if Mozilla goes closed source the already existing forks can continue