macOS keyboard definition location?

That'd be

/System/Library/Keyboard Layouts/AppleKeyboardLayouts.bundle

. Note that this bundle has been optimised for deployment and won’t make a lot of sense from the outside. If you want to create a keychain layout, I believe the current state of the art is defined in Technote 2056 Installable Keyboard Layouts. You should also search for

'KCHR'

and

'uchr'

resources, because a bunch of that info will be useful for historical perspective.

I haven’t actually created a keychain layout in decades — not since we add Dvorak layouts to Mac OS 8.6 — so I don’t have any practical experience with this. I think it still works as documented in TN2056 but I could be wrong.

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