There have been a couple (or maybe one or a few) that started with very little. A lot more had a gift or loan from a parent for enough to buy land or start a business, often that was less than $100,000

As much as Musk's family had money before, his initial big money came from his share of his brother and his city guide software "zip2" which they sold to Compaq for a few hundred million. Lots of people have made more complex or bigger programs with no more wealth than an average middle class family.

Then x.com (the 1999 one, a bank) which became part of PayPal which sold to eBay for $1.5 billion which Musk got a share of

Then he made SpaceX then Tesla* and Tesla made him a billionaire through his ownership of a large part of it

*Tesla was made of Musk's money and A/C Propulsion electric vehicle conversion system. It was incorporated with SpaceX's incorporation papers with the company name changed. A/C propulsion's drivetrain was replaced by a new system after a year of production of the roadster