Check the wikis first - they both are extremely comprehensive and should cover most questions!
Even though I prefer Fabric I recommend making a mod first and getting started with Forge. Trust me when I say converting a mod after you've made one makes so much more sense
That's a great question that requires some decent Java expertise
Its per-case, per-mod, per-version
If you're lucky someone else has done the work for you and ported it to both versions. But in general if a mod is made exclusively for Fabric or Forge, it's gonna stay that way
Getting Fabric and shaders is actually super easy and totally worth it!
It's incredible how easy modding MC has become, not only that - but shaders aren't like the old days where you only get 15fps, it all runs so smooth and great together