Apple TV+ is not its own app, it is a service through the Apple TV app. Apple did say Apple TV would be available for iOS, Mac and "other platforms" but did not indicate that users would be able to stream it from a web browser like Chrome, Firefox, or Microsoft Edge. Apple TV+ will be availble fall 2019. It will include only original Apple shows and movies, and the ability to stream from some Apple partners without leaving the app.
Can I stream Apple TV Plus from my computer/laptop?