It's defined at the browser level. It determines which events fire, and in what order. Which doesn't mean there aren't situations where the OS has some say, but when you touch it tends to pause to see if there is a drag starting, then sets off a shotgun style blast of events including the click event which generally sends you off somewhere you don't want to go. That is unless the developer knows what all to trap and stop and in what way makes sense for that situation. Some devices (like iOS ones) have made some efforts to simulate desktop hover on the first touch but it's really a lawless, non-standardized mess. The one time I had to dig through it all made me hope I don't have to again. MS of all groups is trying to standardize it, and getting high marks for the proposed standard.

We'll see, but touch has more or less killed hover which sucks because it's very useful on the desktop. Though it was already discouraged for accessibility concerns for anyone interested in that deep dark hole.