Apple company is a public company that means its shares publicity available on the market. They have legal responsibility to reveal certain information to their shareholders that mean they have share it with everyone. Apple have an own many company and they are not required to disclose each one even though the information would be available if you were checking their financial reports with enough analysis. if private company has an apple as an investor then they don’t need to disclose information.
As we know that apple must disclose information such as what they know but private companies can go in arrangement with investors. If apple try to start a company as a subsidiary, then it will fall under the same rule as apple does.