How captive.apple.com is used by non-Apple devices?
Source:


  Most hotspots implement customer interaction by using a captive portal, which is a restricted network connection in which all client HTTP requests are redirected to the provider’s web site. The web site can then prompt users to agree to the operator’s terms and conditions, enter payment information, or enter credentials to verify prior payment arrangements.


Apple hosts a landing page and makes a synthetic call to this when you join a network to test if the network is captive. Other vendors have similar portal detection hosts like www.msftncsi.com to detect captive networks, that's all. 

It doesn't matter if the device you're using is a Mac, iPhone, Pixel, or Windows. The "Apple" part is only because they made it much easier for companies to offer wifi to their customers.