As a citizen of a country where prepaid access is the norm, it's just DNS (mis)redirection. People call it "captive portal".
When your quota runs out everything you access resolves to the same address, which is an internal web site where you can buy more data.
And yes, it does not work if you use a custom DNS such as Cloudflare or Google Public DNS, or if you try to reach a HTTPS page. But they don't care, as it's a minority that does this. Android already has support for captive portals, and shows a notification when it detects one.
When your quota runs out everything you access resolves to the same address, which is an internal web site where you can buy more data.
And yes, it does not work if you use a custom DNS such as Cloudflare or Google Public DNS, or if you try to reach a HTTPS page. But they don't care, as it's a minority that does this. Android already has support for captive portals, and shows a notification when it detects one.