Hangouts is Google’s dedicated messaging app, it allows us to make calls, send and receive SMS and access your voicemail ect, now it supports for iPhone X. With it, we can take advantage of the full display of the iPhone X without black bars. And when you need it on your iPhone X, you will need to download your installed copy of hangouts to version 21.0 or later.
And when you have an iPhone X, you can download it from iTunes Store
When you need more information about it, you can check Google website for it.