Will all apps be getting quick reply like iMessage?
Not unless developers implement it, but to my knowledge, developers haven't been given access to that API in the first place.
It has given access to quick to apps , how else do you think Facebook quick reply works
Quick Reply, like in Messages, where you can type a message in the notification banner, is only for Apple apps. All third party apps can do is add action buttons.
False:
https://forums.macrumors.com/threads/1742945/