Basically get lucky. One of the biggest mortgage lenders in my area is a complete dipsh!t, but got started in that business the first time rates went to historic lows. I think it was in the mid-2000's. He made enough money from that to start his own mortgage lending company, and basically hired a bunch of his friends that couldn't get good jobs. Still doing it, and making a ton of cash.
Another friend who is also a dipshit, but a really nice guy, started doing realty in very expensive areas of California. I don't know that he's wealthy, but he's closing about $5M in homes per month, and I assume taking his 2.8% commission on all those sales, which would mean he's clearing about $140k per month, or $1.68M per year.
Last two friends started in real estate development at the perfect time. Started with a single home that they bought for $250k together. At that time, lending was easy to come by, so they got another loan to remodel the heck out of it, and ended up selling it for $550k. Took that profit and bought a bigger place, and sold their 2nd project for $1.5M. They did most of the work themselves on those first two houses. Worked their asses off. But by the time they sold that second house, I think they had something like $500k in profit. Fast forward a couple years, and investors were coming to them to build 200 condos in ski towns, apartments buildings all over the country, custom $3M homes, etc. The last project they told me about, they bought a $14M building.
I think one other poster said he started flipping homes and acquiring rentals. It's a great way to get started. Especially in a college town. Rent demand doesn't go down in college towns. In a recession, the value of the property might go down, but not the rent. You just ride through the recession collecting your rent checks.
In fact, the two real estate developers do a lot of projects for a guy that got started doing that. He owns whole apartment complexes now, and he just got divorced and it came out that he was making $250k per month in rental income.