I'm not so sure about Apple making Wolfram's code open source, they are not really into open source as much as Microsoft or Facebook.

Just for the record, anyone knows about VScode (ms) or React (fb), but who can give me an open source product from Apple and used by a lot of people ?

Edit : typo

Two examples from Apple:

• The Swift Programming Language: https://github.com/apple/swift

• FoundationDB - the open source, distributed, transactional key-value store: https://github.com/apple/foundationdb