Identify square icon with filled triangle up icon in iOS status bar
The icon represents AirPlay, Apple's technology that lets you mirror screen content from one device to another.

The icon indicates an active AirPlay session where your iPhone screen is mirrored on an AirPlay compatible device (Apple TV for e.g.).

You can learn more about AirPlay in the Apple Support article:


How to AirPlay video and mirror your device's screen to Apple TV


Turning off the AirPlay session will remove the icon from the Status bar.