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Ok, so it’s been established in prior posts and confirmed by Apple but wots the step by step process of doing so?
I will have to buy yet another cable (sigh) so I don’t want to get it wrong!
My take is that I would buy an HDMI to go directly out of the external computer with a male thunderbolt on the other end to connect straight into tho iMac.
then turn on the external computer…
does that sound about right?
thanks in anticipation.
Nope. Target display mode was deprecated years ago.
Use your iMac as a display with target display mode - Apple Support
Target Display Mode went away for good when the first 27" 5K Retina iMac came out in Late 2014. Your Late 2015 iMac doesn't have it.
In addition, I do not believe that there is any adapter that will let you convert HDMI video output into Thunderbolt input for a display.
A Mini DisplayPort to HDMI adapter might provide the physical connectors you are looking for, but it would not work for the application you have in mind. Wrong direction and wrong signal type. Such an adapter would be useful for connecting a standalone HDMI monitor or TV to the 2015 iMac – but that's not what you are trying to do.
What is the "other device?"
Target Display Mode (TDM) mythconceptions caused a flood of misinformation when work-at-home pressures affected so many in 2020. Starving web "writers," having neither editorial support nor the most basic research skills, touted TDM as doing things from supporting any known electronic device to parting the Red Sea (well, almost). Even Apple employees were parroting that post-processed horse fodder. Those scribblers could have saved a lot of confusion and expense to others had they taken three minutes to look up the Apple TDM documentation which, by early 2020, showed TDM as dead and buried.
+1 The Late 2015 iMac does not support being used as a display.
Please carefully read the Apple Support article that KiltedTim posted.
+4 Target Display Mode has been dead for a decade now. iMacs are computers, they are NOT external displays for other devices. If you need an external display please buy one.
Using late 2015 iMac as monitor for another device…
