Valve Spring Installed Height Variation

Kern Dog said
How about the question that is circled in blue?

View attachment 1716376753

Do the shims go against the surface of the head with aluminum heads or between the spring and cup?

They go between the head and the spring locator/cup. At least that's how the ones I've had were installed. It makes sense to me since the locator is probably going to be thicker than the shims used. I'd want the thicker piece against the spring.