A company becomes public if it decides to do an Initial Public Offering or IPO and then trade at least one share on the open stock market. These shares are then available to purchase by investors, allowing capital to be raised this way for the public company.
For example…
Nestle is a public company. The food product giant is Swiss-based and was first founded in 1886. Operating in close to two hundred countries, it is one of the most successful and wealthy food product companies in the world. Its shares are traded on the SIX Swiss Exchange and the London Stock Exchange to name a few.