Minecraft Preview

Minecraft Preview is an application players can use to test out beta features from Bedrock Edition. It works as a separate application, rather than the previous system of opt-in beta content inside of the Bedrock Edition game itself.

As of now, it is currently available for limited users on iOS/iPadOS devices, and on Xbox. It will also be released on Windows 10 and Windows 11 within a few weeks.

iOS/iPadOS
Installing Minecraft Preview on iOS/iPadOS requires the user to be enrolled on TestFlight, a beta enrollment application. The app is currently available to a limited amount of users. To get in click here.

Windows 10/11
It has been confirmed that the app will be available on the Microsoft Store if the user have bought Bedrock Edition on it. It has also been confirmed that it will also be integrated into the new Minecraft Launcher for Windows, and be available in Xbox Game Pass.

Xbox
In Xbox you don't need to own Minecraft in Phase 1

It is currently still possible to opt into the beta using the Xbox Insider Hub app for now.

More info here.

Other platforms
It is mentioned in the FAQ that Minecraft Preview is planned to be released on other platforms, with it being stated that the FAQ will be updated once the app is released on said platforms.