How does Google Maps delay showing map until current location is obtained?

[David Goldsmith]

In JavaScript, with <script async defer src="https://maps.googleapis.com/maps/api/js?key=myKey&callback=myCallback"></script> (in the HTML file), I've found that trying to get the user's position before the Map has been instantiated--so that said instantiatiion can be done with said position for center--using navigator.geolocation.getCurrentPosition through the callback, doesn't "work," because navigator.geolocation.getCurrentPosition must return immediately (according to its spec), but before the user's position can be obtained, s/he must first grant permission.  The upshot of this is that, seemingly, one must instantiate the Map with a LatLng constant (or, equivalently, literal), wait for permission to be sought and received, and then call navigator.geolocation.getCurrentPosition and re-center the Map using that result (which I've found to further require the user to perform some action, e.g., clicking on a button, to call Map.setCenter, but then, concerning this further requirement, maybe I'm missing some other way).  

However, if, for example, one launches Google Maps via the "nine squares" icon in the upper right corner of a Google page, the resulting map is automatically centered (roughly) on one's location upon initial presentation: there's no first-you-get-an-arbitrary-center-then-you-have-to-manually-recenter mumbo-jumbo (nor does one have to grant permission for this to work, interestingly).  How does Google Maps achieve all this?  Are they using some means other than navigator.geolocation.getCurrentPosition and/or <script async defer src="https://maps.googleapis.com/maps/api/js?key=myKey&callback=myCallback"></script>?  Are they simply "hiding" an arbitrarily-centered map until the user's location is obtained, then re-centering and "unhiding" the map (which I tried, but couldn't get to work, due to the apparent need for the user to interact in some way to call navigator.geolocation.getCurrentPosition)?  And how are they, seemingly, skipping the permission step?  

(What I've done/learned, so far, on my own to try to figure this out: first, as stated, I confirmed that Google Maps behaves in the way described/desired; then, I ctrl-clicked (I'm a Mac person) on the Google Maps App icon and selected Copy Link Address, obtaining the URL https://maps.google.com/maps?hl=en&tab=gl&authuser=0, which I then manually pasted into the address bar, and hit enter; I then observed that, a few moments after the "correctly-centered" map is displayed, the URL in the address bar automatically changes to https://www.google.com/maps/@<myLat>,<myLon>,13z?hl=en&authuser=0, i.e., one which includes my location's Lat and Lon: clearly, my location is being obtained and used to center the map before the map is displayed.  I also used the Chrome Developer Tools to look for and search what .js files I could find to try to locate navigator.geolocation.getCurrentPosition, to no avail, so either Google gets the user's location some other way, or I just couldn't find the .js file in which this function resides.)

Thanks for your help!