Short answers, pulled from the story.
"Poor Little Fool" by Ricky Nelson was the first number one on the Billboard Hot 100, which launched on the 4th of August 1958. The chart replaced several predecessor charts, including "The Top 100" and the best-seller chart, which both ended in the weeks just before the Hot 100 debuted.
The country singles chart was the first Billboard chart to use SoundScan and Broadcast Data Systems, switching in 1990. The Hot 100 and the R&B chart followed in 1991. Before SoundScan, Billboard relied on manual reports from radio stations and stores, which allowed labels to influence chart positions by stopping promotion.
Before December 1998, a song had to be released as a commercial single to qualify for the Hot 100. Major labels sometimes withheld popular songs from single release to encourage album sales. "Torn" by Natalie Imbruglia peaked at 42 only after the policy changed. The Goo Goo Dolls' "Iris", No Doubt's "Don't Speak", and Sugar Ray's "Fly" were among other songs that had been excluded under the old rule.
The Hot 100 currently combines singles sales, radio airplay, digital downloads, and streaming activity, including data from YouTube and other video platforms. The sales and streaming reporting period runs on a Friday-to-Thursday cycle since July 2015, while radio airplay follows a Monday-to-Sunday cycle. Charts are released each Tuesday with an issue date the following Saturday.
The Beatles ranked first on Billboard's 2015 ranking of the 125 best-performing artists over the 52 years from 1963 to 2015. The Rolling Stones ranked second, followed by Elton John, Mariah Carey, and Madonna. The rankings were based on combined point totals from both the Hot 100 and the Billboard 200.
The "Best Selling Popular Record Albums" chart first ran on the 24th of March 1945, with The King Cole Trio at number one. The chart started with ten albums before shrinking to five in the following weeks, expanding again over time, and eventually becoming the 200-album Billboard 200 on the 13th of May 1967.