MeowDisplay app lets iPhones and iPads act as a Mac's display, with Android in beta
MeowDisplay is a free, open-source (GPL-3.0) application that reverses the usual screen-mirroring setup, letting an iPhone or iPad serve as an external or primary display for a Mac via a software-created virtual screen rather than a physical monitor. The Mac sender app (stable v2.0.0) requires macOS 14 or later, iOS/iPadOS receivers need version 16.4 or later, and an Android receiver is currently in closed beta testing on Google Play; a Mac-to-Mac receiving mode also exists but is not yet fully verified.