Both. Unless you have enough money to start your own private equity firm, you could invest indirectly by investing in the firm itself, e.g., Berkshire Hathaway. There are also Mutual Funds and ETFs that invest in private companies, which is another way to gain exposure. Note that these have investment minimums. Don't hold me to the fire for it, but unless I misremember, Vanguard offers one for a 700k minimum. Many options require one to be an Accredited Investor. To qualify, one has to have ~200k in income, assets over 1M, and/or work in the financial industry.

Investing in private companies isn't meant for the average retail investor, and unless one invests as part of a larger fund, they'd likely be disinterested in smaller investment amounts.