How can I use my iMac as a display connected to my Mac Pro?

The very restrictive OS and hardware requirements for the now-deprecated Target Display Mode are spelled out here:

Use your iMac as a display with target display mode - Apple Support

Note Grant's point about staying with Catalina on the Mac Pro is included there.

Remember this is dead tech. Apple start the deprecation of TDM in Late 2014 and threw the last shovel of dirt in the grave in early 2020. Any standard external display—even the cheap ones—are miles ahead of TDM in set-up, video option availability, and cost of running than using an iMac.

The others have already made the point about the 2019 Mac Pro needing to be on Catalina to use TDM. You would also need $80+ in specialized Thunderbolt connection hardware: an Apple Thunderbolt 3-to-2 adapter and a $30+ Thunderbolt 1/2 cable.

It's not worth hobbling that 2019 Mac Pro with Catalina, and spending $80+ on Thunderbolt connection hardware, just to avoid buying a good 24" 1920x1080, 27" 2560x1440, or 27" 3840x2160 pixel monitor. That old iMac might have a good-quality screen – but even if it is a 27" screen, it is something like 10–11 years old and it is not a Retina one. It makes more sense to spend money on a good-quality 27" 2.5K or 4K monitor that will be more convenient and will consume less electricity.

1. Yes, but see #2.

2. If you're using Target Display Mode, yes. If you're using Virtual Network Computing, no.

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1. It doesn't matter whether you use Thunderbolt 1 or 2.

2. You'd need to stay on Catalina; it won't work with any higher OS.

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