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Sidecar is only possible if your Mac and iPad are fairly new models. Also, it doesn't allow one to use e.g. another MacBook or iMac as a screen.

I use YAM display[1] with a 2012 macbook and an ipad mini from 2015, which works great!

I don't think you can use another mac as a second screen though.

Also check out Luna Display if you want to use another Mac as a display: https://astropad.com/product/lunadisplay/