Apple implemented both their own as well as third party methods in their due diligence.
See page 4
Specific statement:
"Based on its assessment of survey responses received from suppliers, Apple implemented due diligence measures with 233 smelters and refiners that were identified as potential sources of Subject Minerals in Appleās products. These due diligence measures included engaging directly with unverified smelters and refiners to drive them to comply with the CFSP or an equivalent third-party audit program. Apple personnel also conducted 38 visits with smelters and refiners and traders, and established action plans with particular smelters and refiners, to prepare them to undergo conflict minerals audits. Additionally, Apple commissioned a third-party review of publicly available information on the identified smelters and refiners reported to Apple, providing additional information to determine the country of origin of those smelters that have not gone through the CFSP or an equivalent independent third-party audit."