I want to use my iMac 2017 running "Mojave" as a second monitor for my MacBook Pro mid 2012 running "Yosemite".
I can't find any tutorials for such age difference. Any help or tips are very appreciated.
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I don't think there's a built-in way to accomplish what you're going for. But there are some products (hardware and/or software) you can purchase to help.
iMacs used to have a mode wherein they could act as a secondary external display. Even though current iMacs no longer offer that functionality, this product provides functionality to turn a Mac into a secondary display. I've heard very good things about it, but I've never tried it myself. It requires a hardware dongle plugged into the source Mac (where the video is coming from): https://lunadisplay.com/pages/meet-mac-to-mac-mode
And here's another that advertises similar functionality. Unlike Luna Display above, it doesn't require a hardware dongle: https://www.duetdisplay.com/air
(I don't have any relationship with either company or product.)