"the cost of changing and reconfiguring the internal walls of the building?"

Maybe.

"what QIP expenses actually qualify?"

Think of the difference in 15 year property and 39 year property. For instance, closets and cabinets are not structural.

Here are some resources.

https://www.irs.gov/newsroom/new-rules-and-limitations-for-depreciation-and-expensing-under-the-tax-...

https://www.irs.gov/newsroom/additional-first-year-depreciation-deduction-bonus-faq

https://www.jdsupra.com/legalnews/irs-releases-rev-proc-2020-25-qualified-14613/

https://www.irs.gov/pub/irs-drop/rp-20-25.pdf