SAN FRANCISCO — Technology start-ups worth $1 billion, once as rare as unicorns, are now plentiful enough and old enough that there’s a new generation behind them — one that looks very different.

Silicon Valley’s current crop of highly valued tech start-ups, which include now-household names like Uber and Airbnb, all benefited from the spread of smartphones and cheap cloud computing. Many of these companies built global empires by simply taking existing businesses — like taxis, food delivery and hotels — and making them mobile. Some of the start-ups became giants: Uber, for instance, may reach a $120 billion valuation this year.