Just out of curiosity I asked on ChatGPT and here is its answer:
"

In the United States, according to the Securities and Exchange Commission (SEC) rules, a large public company must report its ownership stake in another company if it reaches 5% or more of the outstanding shares of that company. This reporting requirement is triggered under the Securities Exchange Act of 1934, specifically under Section 13(d).

So, if the large public company buys shares in a smaller company and reaches 5% ownership, it would typically need to file a Schedule 13D or Schedule 13G with the SEC to disclose this ownership stake.

If the large public company holds less than 5% of the smaller company's shares, it generally doesn't have a requirement to report this ownership. However, there may be other considerations such as materiality and whether the purchase is considered a "strategic investment" that could impact the joint venture. It's always a good idea to consult legal and financial experts when making such investments to ensure compliance with all relevant regulations and agreements."