First Responder, but this happens at the hardware/firmware level and is not strictly a function of iw.
There's no standard that I am aware of, just common practice. Most things network default to first responder, So most firmware follows suit. Typical firmware can not be tuned, but I have seen a very few that can be. usually in diagnostic or very high end cards. Most (even high end) just go with the de facto first responder. You can however use the mac or BSSID to connect to a specific AP if that is your goal. Most any card will support that. Keep in mind ESSID is for "entire networks" (groups of BSSIDs) so you should be able to float around APs that make up that ESS w/ out intervention (in theory).