How to send notification to my app from my website manually

So one of my friend made an app. And he owns a free website. And I don't know how but he can send push notification to his app even to certain device by just editing a php file stored in his website. When I ask him, he doesn't wanna tell me the process. He didn't used any external method like firebase,onesignal etc. He did it all manually. He said that he just needed to file. One php file for sending notification title and message and one java for retrieving those text's and showing notification. Can anyone give me any clue/suggestion /tutorial of how he did that? This is his website and the app's name is webview, a browser.