Material composition is a varied answer because of the thousands of individual components and suppliers. Specifications are supplied in PO’s etc. and compliance is expected per ISO standards and inspections to verify. Apple supplies this information to the public when it’s necessary due to regional, state or country regulations or laws. Think of the California cancer warning on products available for sale in US.
However, there is not an absolute right for consumers to know everything. Think, patented components, trade secrets etc. that are also protected by law. This is a request best directed toward Apple corporate and Apple Support is the best way to examine and answer the Apple product owners request.