Investment holding companies refer to companies that mainly own and manage equity investments in other companies. The list of such companies is considered proprietary information, and it’s important to obtain it through proper legal channels. There are a few options to legally access investment holding companies information:

Check public information of listed investment holding companies
Many investment holding companies are publicly traded on stock exchanges. You can find their financial statements, annual reports, and shareholding structure information from credible public sources. For example, Berkshire Hathaway, an American investment holding conglomerate led by Warren Buffett, regularly publishes its equity holdings details to comply with disclosure requirements.
Commission research reports from professional agencies
There are many professional market research firms that compile and sell reports on major investment holding companies globally or within certain geographies or industries. You can legally purchase such reports that aggregate information from public filings and other sources.
Engage services from law/accounting firms with M&A practices
Many large multi-national law practices or accounting firms have dedicated M&A and deal advisory teams supporting investment holding companies. You may engage their professional services to legally obtain tailored information on such firms catered to your specific needs.
Network with practitioners in investment management industry
You can connect with investment holding company executives, investment managers, lawyers and other professionals who work closely in this domain. Building trusted relationships and networks can help you gather useful insights through legal means.
There are proper legal channels to access investment holding companies information such as public disclosures, commissioned reports, professional services and industry networks. One should not obtain any proprietary data through unethical means.