I don't think HTTPS is the default automatically. When I create a new Chrome profile, then type example.com into the url bar and hit enter, I go to http://example.com . Then when I type https://example.com twice (each time hitting enter) then type example.com and hit enter, I go to https://example.com . So I think it might be whichever is more common in your history.
Firefox behaves very similarly, except I don't have to visit https://example.com twice before it becomes the default, I just have to visit it once.
HTTPS Everywhere uses built-in rules, not heuristics, to determine when to rewrite an HTTP request to HTTPS. There does not appear to be a rule for example.com, so HTTPS Everywhere does not cause a rewrite.
Firefox behaves very similarly, except I don't have to visit https://example.com twice before it becomes the default, I just have to visit it once.