Mr. Biden’s biggest benefactors in the second quarter of 2020, when he became the presumptive Democratic presidential nominee, represent a who’s who of billionaires and influencers in Silicon Valley, Hollywood, Wall Street and beyond. Among those who gave at least $500,000 were Laurene Powell Jobs, the philanthropist and widow of Steve Jobs; Meg Whitman, a former Republican candidate for governor of California and now the chief executive of the streaming company Quibi; George Soros, the billionaire progressive financier; Jeffrey Katzenberg, the Hollywood producer; and Dustin Moskovitz, a co-founder of Facebook.