That’s much harder. If it’s a big private company like OpenAI, you need a minimum 50k investment. For smaller companies, the number is much higher I assume.

Yes.

Can you really invest only 50k into OpenAI? That seems low to me.

However, some small companies are okay with small investments. You have to negotiate privately, that's just how it is. If it was a simple, uniform process, they'd be on the stock market.