> I have no way to distinguish if I actually am affecting this "world." For all I know someone is just modeling stuff, and when I draw a "fence" it goes and finds a random fence and says "here's the fence you made, yay for you!"

I have a feeling that's exactly what's going on here because I drew a small bush and it said "Thank you, the world needed 4 of those!".

Also take a look at where most of the trees were created at. Shoreline Amphitheatre. :)