Short answers, pulled from the story.
The double bass is the largest and lowest-pitched chordophone in the modern symphony orchestra. Unlike the violin, viola, and cello, which are tuned in fifths, the double bass is tuned in fourths (E1, A1, D2, G2), and its construction combines features of both the violin family and the older viol family.
The double bass descends partly from the violone, the largest member of the viol family, which was tuned in fourths. It is the only modern bowed string instrument that retains this tuning, in contrast to the violin, viola, and cello, which are all tuned in fifths.
The French bow is held with the palm facing the bass, similar to how smaller string instruments are bowed, and was not widely popularized until 19th-century virtuoso Giovanni Bottesini adopted it. The German bow is the older design, descending from viol family tradition, held with the palm angled upwards. Modern players in major orchestras use both.
Domenico Dragonetti was a prominent figure who performed with the Philharmonic Society of London and was an acquaintance of both Haydn and Beethoven. Giovanni Bottesini was called the Paganini of the double bass in the 19th century. Serge Koussevitzky popularized the bass as a solo instrument in the early 20th century, and Gary Karr was the leading proponent from the 1960s through the end of the century.
A C extension is an extra section of fingerboard mounted on the head of the instrument that extends the lowest string's range downward by four semitones to C1. Most professional orchestral players use one because composers including Wagner, Mahler, and Prokofiev wrote notes below the instrument's standard low E, and doubling the cello part an octave lower often requires descent to C.
According to professor Larry Hurst, the appearance of the overwound gut string in the 1650s saved the instrument. Before that development, the thickness required for gut strings at the bass's lowest pitches made those strings almost unplayable. Overwound strings attain low notes within a smaller overall diameter, making the instrument practicable at scale.