The HTML file in this repo is a set of web links normally used to detect captive portals on WiFi networks. Add this file to your phone and bookmark it in a mobile browser. If you connect to a WiFi network that you know has a captive portal but the phone fails to bring up the captive portal web page, try the links in this file to see if you can access the captive portal this way.
The Google and Apple links are default captive portal detector URLs on various devices.
The other links come from sources such as this Reddit post.