Short answers, pulled from the story.
The harpsichord emerged in the late 14th century as a small instrument with a high pitch. Early makers crafted lightweight cases with thin walls and used brass strings arranged in two choirs.
The Ruckers family created heavier instruments with powerful tones and steel treble stringing starting around 1580. These Flemish models became the standard for later construction across Europe.
Depressing a key raises its back end within the instrument to lift one or more jacks holding a plectrum made from quill or plastic. This plectrum plucks a single string under tension on a soundboard mounted in a wooden case before returning to rest position when released.
William Byrd composed numerous works during the whole Baroque era while François Couperin created highly characteristic solo works compiled into four books of ordres. Domenico Scarlatti wrote 555 harpsichord sonatas and Georg Friedrich Händel composed numerous suites alongside J.S. Bach.
Wolfgang Amade Mozart and Joseph Haydn both wrote harpsichord music earlier in their careers before the instrument was largely supplanted by the piano starting roughly in the late 1770s. Through the 19th century the harpsichord was almost completely supplanted by the piano until Arnold Dolmetsch influenced a movement that brought it back during the 20th century.