Where's the code for the actual HyperCard and building of the .img? I only see the python validator in the repo.

The stack is the code. You can view it directly for each button or examine the per-page script. As far as I know there isn't a compiler that lets you write standalone code and turn it into a stack. The stacks are dropped into Disk Copy disk images to preserve their resource forks. Both modern macOS and Git both strip resource forks, so the disk image is the only reliable container for distribution.

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tty456 an hour ago | parent [-]

So a hypercard is compiled machine code of button clicks and key presses? Weird. I guess that could be macro'd somehow

	▲	hammer32 an hour ago \| parent [-]
		HyperTalk is an interpreted scripting language. The scripts are stored as plain text inside the stack and interpreted at runtime. It's kind of like a Visual Basic form where the UI and the code live in the same file. You can open any script, read it, edit it and immediately run the newly edited script.