How did they get it?

Got lucky on Yahoo! Japan in the 1990s.

Wikipedia says Alibaba was their big win. And then they convinced the Saudis to let them bet with Saudi money. They also have decent net income trends:

https://companiesmarketcap.com/softbank/earnings/

Either way, seems like enough of their bets work out, so far.

https://blogs.wsj.com/digits/2014/09/19/softbanks-alibaba-al...