How much of a movie's sound effects is a foley artist responsible for? Are all non-dialogue sounds rendered by them? How is this decided?
Foley Artists are responsible for a lot of different types of sounds; mostly natural sounds like footsteps and the sound that certain clothes make while moving in them, but pretty much anything that you can fit into a foley studio. In addition to Foley Artists there are also Sound FX Designers whose job is to create the sounds that can't be naturally or easily recorded.
The following text (which I grabbed from marblehead.net) does a pretty good job of describing who's responsible for what:
When it comes to how much of a film's sound effects that are actually added during post-production, an excerpt from an article featuring an interview with Oscar-winning sound editor Richard King tells that it's almost all of them. Though, it most likely differs from scene to scene, and movie to movie.