He gained the vast majority of his riches from Tesla stock. According to this page outlining Tesla ownership, he owns about 20% of the company and that 20% alone is worth about $280 billion right now. How is it considered worth that much? Well, long story short, the stock market is detached from reality. People like to claim that it's rational but we're talking about an automotive company worth more than the next 62 or so automotive companies combined even though it produces a fraction of the vehicles of the others.

The only remaining argument to justify the value is future potential and some claim the stock market is "future looking" as if it can tell the future. What lies in the future for Tesla is apparently robotaxis, robots, and some other AI-based vaporware. People also claim that the batteries are a huge advantage for Tesla, but Tesla doesn't make the cells and they're not involved in the chemistries, they're just assembling cells into batteries. This is where China would absolutely destroy them, but of course Musk got the government to protect his ass by slapping 100% tariffs on Chinese EVs.

Speaking of the government, he also gets lucrative contracts from the government for SpaceX. He's one of the biggest welfare queens of all time. Tesla was saved early on by carbon tax credits (which was the only money maker for Tesla for most of its life), another policy government that built up Musk to what he is today. He likes to think of himself as some rugged individual capitalist, but he'd be nothing without the billions in government support.