Short answers, pulled from the story.
The Billboard Hot 100 premiered on the 4th of August 1958 with Poor Little Fool by Ricky Nelson as its first number one song. This chart replaced previous separate metrics to become a single all-genre singles chart for the industry.
Chart rankings rely on sales data from Nielsen SoundScan including retail and digital formats alongside online streaming activity from sources like AOL Music and Yahoo! Music. Radio airplay audience impressions are measured by Nielsen BDS across approximately 1,000 stations and computed by cross-referencing exact times of radio airplay with Arbitron listener data.
The modified tracking schedule took effect in the issue dated the 25th of July 2015 following changes to conform to a new Global Release Date shifting United States product release from Tuesdays to Fridays. Radio airplay adjusted to align with other metrics effective with the chart dated the 17th of July 2021 changing the tracking period from Monday to Sunday to Friday through Thursday.
Only five artists had Hot 100 Top 40 hits in each four decades from the 1980s through the 2010s including Michael Jackson Madonna Weird Al Yankovic U2 and Kenny G. Mariah Carey became the first artist with a number-one single in four different decades.
The Canadian Hot 100 launched on the 16th of June 2007 using sales and airplay tracking compiled by Nielsen SoundScan and Broadcast Data Systems. The Vietnamese edition known as Billboard Vietnam Hot 100 launched on the 14th of January 2022 following global expansion efforts.