How do Wikipedia’s processes for creating and modifying articles ever lead to high-quality results? In other words, since anyone can easily edit Wikipedia, how is it that good (and usually accurate) content emerges?

I think the primary factor at play that helps produce generally solid results is the ratio of "good" contributors to "bad" contributors.  By "bad" I don't necessarily mean contributors with bad intentions.  Those folks certainly qualify as bad.  But I'm also referring to those contributors who provide erroneous or inaccurate information with no malice intended.  The bottom line is that the results of the contribution are of poor quality.  As long as there are significantly more contributors interested in accuracy and factual information than there are contributors that deteriorate the quality of entries, wikipedia will generally be an accurate encyclopedia.  Therefore wikipedia essentially depends on that ratio favoring the "good" contributors.  What that ratio actually needs to be to ensure quality entries, is a good question!