Short answers, pulled from the story.
The origin of scat singing is disputed. Jazz pianist Jelly Roll Morton credited Joe Sims of Vicksburg, Mississippi, as the first to perform a scat number in the United States. Morton also said he and Tony Jackson were using scat in 1906 and 1907. Louis Armstrong's 1926 recording of "Heebie Jeebies" is often cited as a turning point, though many earlier recordings predate it.
Scat singing uses wordless vocables and nonsense syllables, with the singer improvising melodies using the voice as an instrument rather than a speaking medium. Vocalese is different: it uses recognizable lyrics that are sung to pre-existing instrumental solos.
Armstrong claimed his sheet music fell off the stand mid-recording in February 1926. Not knowing the lyrics, he improvised wordless gibberish to fill the performance, expecting the take to be discarded. The engineers instead chose that take for release, and the song became a national bestseller.
Fitzgerald's scat performances followed a consistent structure: the same tempo, an opening chorus of the straight lyric, a specialty chorus, and then the scat itself. She developed her style partly by imitating Connee Boswell as a young girl. In her 1960 live recording of "How High the Moon" in Berlin, she quoted more than a dozen songs including "The Peanut Vendor" and "Smoke Gets in Your Eyes."
Hip hop artists and rappers use scat singing to develop the rhythmic skeleton of their raps before writing lyrics. Tajai of Souls of Mischief described making a scat skeleton of a flow and then filling in words. Rapper Tech N9ne has demonstrated this process, and Eazy-E used it extensively in composing "Eazy Street."
Scatman John was the stage name of jazz artist John Paul Larkin, who fused scat singing with pop music and eurodance in the mid-1990s. He scored a worldwide hit in 1994 with "Scatman (Ski Ba Bop Ba Dop Bop)," briefly renewing broad public interest in the genre.