By own knowledge and experience.
There is the following service: link that modifying the configuration of wifi and putting one of their dns on the iphone opens a captive portal with the following interface ui.iclouddnsbypass.com (to see it in chrome you have to give it to inspect and select view of mobile), I do not know much about networks I am more of web development, I have seen tutorials of captive portals but only in local network, my question is how they manage to do this and how can I put my own website since I am developing a project called miniOS Web that is better.
here is a video of how it works: link
I would greatly appreciate your advice and criticism about this project to be carried out