How do I setup a basic Hello World project in XCode

That part I ignore... since I need something to deploy it on the end user's system anyway. That at least you can do manually, once you've worked out where to put plug-ins. It changed at DC, formerly it was inside the app bundle, now it's a folder under Application Support somewhere. The script may tell you. It is different for the continuous and classic version.

I downloaded the SDK and reset the target SDK (macOS) to 10.13. I've got most things working now (I think), except the final stage of the build, where a script is called to copy the result (I think) to where Adobe finds plugins.

It can be hard to find the right XCode, and I’m still using the first DC SDK. Let’s hear from someone who has a compatible release. But first, did you download the relevant MacOS SDK and reset to 10.8?

9.2? The SDK for Acrobat 9? That is many years obsolete.

I’m saying that you should not make difficulties when starting with plugins. It’s hard enough without also trying to solve problems of system and IDE compatibility with a development environment you don’t yet understand. Once you have a basic plugin going you can try to change things to suit you better.

10.8 may not be needed: that’s just the okdest targeted system, for which you load the SDK in the usual way.

(And you should also use the specific OS version for which the SDK was designed. I use Parallels to do that).

Don't try to get your own code to compile until you have the starter project compiled and working. Is it working? You probably should not update the MacOS framework, though, not unless it is working well and you want to experiment. Just install the old SDK.

Which version of the SDK do you have? What is the difficulty in getting hold of older versions of XCode?

If it's any consolation, getting the first project to compile, load, and become a plug-in is usually a painful experience. Crucially DO NOT START AGAIN or try to make a complete project from scratch. Everything in the starter project is there for a reason.

PiMain.c doesn't need to be understood or looked at but it does need to be included in every project. This archaic stuff is simply how it works; you are not writing an executable module, but something that Acrobat has to be able to load using a standard handshaking method, and which has to set up a programming environment where your code calls methods in Acrobat without using any kind of standard linkage.

So, using an existing project is the key. Make sure you are using the version of XCode for which your SDK version is targeted.

The essence is that

1. Acrobat will call your plug-in and run startup code 

2. This starter code will register callbacks

3. Acrobat will call your callbacks when events happen

You cannot force this to any other model, like using main() to call API calls, or make an executable. You need to study the events and callbacks available to determine how and when your required code will run.