There are a few ways to invest in non-public investments. Here are two examples.
https://www.angelcapitalassociation.org/ . You can find an angel investment group and buy shares of start-ups or funds that invest in start-ups.
HIIVE is for pre-IPO companies that have private shares:
https://www.hiive.com/securities/spacex-stock (picked SpaceX as I am very bullish)
I'm sure I could find more but would agree that starting a business and/or real estate are two of the best ways to grow your investments.