The American music industry uses The Billboard Hot 100 as the prime record chart for U.S. singles. The chart is updated weekly (now on Mondays).
How does a song reach #1 on the Billboard Hot 100?
It is based on a proportion of sales (digital & physical), online streaming and radio play.
All songs are sent through these three charts and then averaged out to get final, overall score. And that score is what you see as the Billboard Hot 100.
However, rules change, so the proportion might be revised again. It already has. Online streaming is the newbie in the proportion. Yet nowadays, where would we be without it?