Microsoft was founded in 1975 and put a computer in the average home, which had not previously seemed like a sensible ambition. Bill Gates co-founded it, ran it as it became dominant, and spent years as the richest person alive.
Dominance drew fire. A landmark antitrust case tested how far a software monopoly could go, and the answer reshaped the industry. In the 2000s he stepped back from running the company to do something else with the money.
The second act was the harder one: applying a builder's rigour to problems that resist quick wins.
The Bill & Melinda Gates Foundation put vast sums into global health, disease eradication and education, and more recently into climate and clean energy. The approach is recognisably an engineer's: measure everything, fund what the data supports, keep going after problems that resist quick wins.
He also co-founded the Giving Pledge, which asks the very wealthy to commit most of their fortunes to philanthropy — and made that a public commitment rather than a private one. The second act changed what a technology fortune is understood to be for.