All the above, fortunately it can be done simply with a roll of railway grass and a camera phone. Best backdrops are natural as it sets the tone for the model. If you prefer, just about anything can be printed onto A3. I have about 3/4 ground scenarios for this, 2 with railway grass rolls glued on. Rather than thousands of words.. in brief.
Natural backdrops, mist/fog can be useful.
Action shots, even wind or a fan has advantages.
Angles and shadows can make all the difference.
Night scenes can be made even without a backdrop.
Even daytime dioramas don't necessarily need a backdrop.
I'd say muck around and find out what works for you. You'll eventually find your own style.