there are hundreds because they got blocked.
android phones actively retry connectivity checks every 3 seconds until a successful connection is achieved, then it ramps down to a check every 5 minutes, and the default server is google's. (this functionality is used for the little exclamation mark next to the wifi icon if there's no internet connection)
no data is sent along that request (it's just a GET request), not even useragent etc (the only thing google gets is the source of the request, aka the ip address, which is basically meaningless if it's not associated with any other data)
you should actually be able to point that domain to any ip that responds with empty body + http 204 code to /generate_204 and it should work as expected