Some websites (but only few) are written in HTML. But all websites produce HTML.
An analogy: ChatGPT is not written in English language. But it produces English language very well.
Yes, mobile apps can be written in HTML. In fact, there are a number of frameworks and tools that make it possible to build mobile apps with HTML, CSS, and JavaScript. These frameworks allow you to create a web app that can be packaged and distributed as a native app.
Mobile apps can be written in HTML, but it’s not the only approach. Mobile apps can also be developed using native languages like Java or Kotlin for Android and Swift or Objective-C for iOS. Additionally, hybrid app development frameworks like React Native and Flutter enable developers to use HTML, CSS, and JavaScript to build cross-platform apps.