It is "very" simple, you either wait for IPO, or you pay a premium to get in a company way before IPO
Private equity investment is "extremely" risky, as you have no way to truly value a company. So, it is all based on faith, such as do you trust Elon Musk and his spaceX or xAI, or do you trust OpenAI.
Definitely not a good idea to invest pre-IPO if you dont have faith in the company