use the Macbook screen as a second screen for another Macbook
From my Macbook Pro 16 (2019) I want to use the screen of a Macbook Pro 15 (late 2013) as a second screen. How can I do that?
MacBook Pro 16″, macOS 14.1
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The 15" Late 2013 MacBook Pro doesn't have a video input port, and doesn't support Target Display Mode. (The only Macs that ever supported Target Display Mode were some old iMacs.)
It also is far too old to act as an AirPlay Receiver for AirPlay to Mac.
You're best off looking for a real standalone hardware monitor.