Is it possible to AirPlay from Mac to iPhone?
No, iOS on the iPhone does not natively support receiving an AirPlay display mirroring connection.
You can use third party iOS apps that employ other types of protocols with accompanying macOS apps that allow you to share the contents of windows from the macOS computer to the iOS device. AirDisplay, that you mentioned, is an example of this.
It is however not native AirPlay and thus has different characteristics. In general, I think it is fair to say that it is a clunkier experience.