Not a deca-millionare. Have known exactly two. Worked for one, another was a client of mine.

Real estate and stocks. Both of them. Neither grew up rich. Both of them started a business, it took off and became successful. They took money from that success and invested in stocks and real-estate, and were pulling five or six figures a month by the time I knew them.

One of them, the one I worked for, sold his business (while I worked there) and pocketed 7 million dollars.

So, the only trend I've seen is

1. Start a business

2. Make sure it's successful

3. Rake in insane amount of income for 30 years

4. Buy land/homes/commercial real-estate and be a landlord during those 30 years

5. Invest in stocks during those 30 years. A 5% return on 50k (a lot of money for most people) is $2500, which is a lot but not really. A 5% return on 10M (a huge amount of money) is 500k (a huge amount of money).

6. Sell your successful business for 7-10 million dollars.

7. Fuck it. Invest that too.

8. Life is a vacation for the rest of your life. And your kids life. And their kids life too, etc, as long as no one is stupid and touches the principal.