Tapping this new button in Chrome Custom Tabs will take you straight to your main browser

Chrome's Custom Tabs are a popular choice for Android apps to display web-based content within their own interface. It offers a browser-like experience without the need for third-party apps to include an entire browser in their code just to load websites, and according to Google, its extra features like password and history sync make it better at preventing users from "abandoning the app" when compared to using Android's default WebView to render the same web pages.