The "Server" must be active on the Pi for the "Viewer" to connect to it. It is pre-installed on official Raspberry Pi OS versions but disabled by default.
Search for "VNC Viewer" in the Apple App Store or Google Play Store. download vnc viewer for raspberry pi
To control your Raspberry Pi from another device, you must download the VNC Viewer application for that device’s operating system. The "Server" must be active on the Pi
Download the installer directly from the RealVNC Viewer Download Page . download vnc viewer for raspberry pi