iMac late 2015 as monitor

Your iMac doesn't support Target Display Mode.

Your MacBook Pro doesn't support using any iMac that is running in Target Display Mode.

[ Details here: Use your iMac as a display with target display mode - Apple Support ]

Your iMac also isn't new enough to be an AirPlay Receiver for AirPlay to Mac (using the built-in support in Monterey and later, as opposed to some third-party application).

[ Details here: System requirements for Continuity features on Apple devices - Apple Support ]

no, well no if you mean by using the old taget display but there are other ways

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Use iMac as monitor: 5 Solutions - 9to5Mac

but mind you a 27" normal monitor would use a fraction of the power of an imac which would be using full running computer watt power while acting as a display