On the Mac: Screen Sharing
Every Mac ships with an app literally named Screen Sharing. Apple keeps it out of the Applications folder, so most people never see it. Open Spotlight, type "Screen Sharing", and it appears. macOS Sequoia gave it a proper window with a list of recent Macs and a high-performance mode for Apple silicon.
To be visible to it, the other Mac needs Screen Sharing or Remote Management switched on under System Settings, General, Sharing. Both Macs on the same network, or signed into iCloud, and you're in. It is free, encrypted, and Mac-to-Mac only.
On the iPhone: SharePlay
There is no app icon for this either. iPhone screen sharing is a button inside FaceTime: start a call, tap SharePlay, choose Share My Screen. The person on the other end sees your iPhone, and on iOS 18 they can draw on it or take remote control with your permission. Everything about it points outward: your phone's screen, shown to someone else.
The full rundown of what the iPhone can and can't share is in Screen sharing on iPhone.
The gap: your Mac's screen on your iPhone
Put the two together and you get a hole in the lineup. The Mac can share to another Mac. The iPhone can share to a person or, via AirPlay, to a Mac or TV. Nothing from Apple shows a Mac's screen on an iPhone, and there's no Apple Remote Desktop for iPhone either.
Third parties fill it two ways. Full remote desktop clients such as Screens connect to the Mac's Screen Sharing service over VNC and mirror the whole desktop, shrunk to phone size. We compare seven of them.
Apperture takes the other route. Its free Mac host streams one app's window, not the desktop, to the Apperture iOS app on your iPhone or iPad. The window fills the phone at the phone's aspect ratio. Taps, scrolls, and the iOS keyboard drive the Mac app directly, and the connection stays peer-to-peer on your own network with no relay. It's the closest thing to the Apple screen share app people go looking for.
Side-by-side details are in Apperture vs macOS Screen Sharing. Setup is about five minutes: here's the guide.
Common questions
Where is the Screen Sharing app on a Mac?
It lives at /System/Library/CoreServices/Applications/Screen Sharing.app. The quickest way to open it is Spotlight: press Command-Space and type Screen Sharing. You can also connect from Finder with Go > Connect to Server and a vnc:// address.
Is there an Apple screen share app for iPhone?
For sharing your iPhone's screen, yes: it's built into FaceTime as SharePlay. For viewing a Mac on your iPhone, no. Apple's Screen Sharing and Apple Remote Desktop only run on a Mac.
Can I download Apple Screen Sharing from the App Store?
No, and you don't need to on a Mac; it's part of macOS. It isn't on the iOS App Store at all because there is no iPhone version.
What app shows my Mac's screen on my iPhone?
A third-party one. Screens and Jump Desktop mirror the full desktop over VNC. Apperture streams a single Mac app's window to your iPhone or iPad with touch and keyboard control, peer-to-peer with no cloud relay.