Short answers, pulled from the story.
Arnold Schoenberg developed the twelve-tone technique in 1921 after several years of research. He first described this method privately to associates in 1923 and wrote his first large-scale work using it, the Wind Quintet Op. 26 between 1923 and 1924.
Pierre Schaeffer began exploring technology applications in musique concrète during the 1940s and 50s. Recording technology made all sounds available for potential use as musical material while magnetic tape availability increased significantly during the 1950s allowing composers to record and manipulate sounds.
Igor Stravinsky drew primitivism into his early career through The Rite of Spring alongside Chout by Sergei Prokofiev. This work represents a key moment where composers returned to the past for inspiration after the First World War while also embracing new forms like neoclassicism with Pulcinella and Symphony of Psalms.
John Cage introduced elements of chance into his music starting from the early 1950s and coined the term experimental music to describe works producing unpredictable results. His approach included techniques such as Cartridge Music which served as an early example of live electronic music using live electronic sounds within performance rather than preprocessed overdubs.
Charles Ives Julián Carrillo Alois Hába John Foulds Ivan Wyschnegradsky Harry Partch and Mildred Couper all explored microtones that are intervals smaller than a semitone. These intervals proved difficult for instruments like piano or organ to produce without retuning during the development of modernist styles.