iPad as 2nd screen
Can you use an iPad as a 2nd monitor using cables instead, without downloading any apps?
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An iPad does not natively support external video input - or use as a connected monitor.
Some models of iPad can be used with a compatible Mac - to extend the Mac display to the iPad using the Catalina/iPadOS Sidecar feature.
https://support.apple.com/en-gb/HT210380#systemrequirements
An alternative third-party product, Duet, can be used to extend PC or Mac display to iPad:
I hope this information is helpful to you.