Short answers, pulled from the story.
Elector Charles III Philip moved his court from Heidelberg to the new city of Mannheim in 1720. This relocation established the foundation for what would become a premier European musical force.
Johann Stamitz became director of the orchestra by 1750 after being hired under Kapellmeister Carlo Grua in 1741 or 1742. He is recognized as the father of the school while other key members included Franz Xaver Richter, Ignaz Holzbauer, Carl Stamitz, Franz Ignaz Beck, Ignaz Fränzl, and Christian Cannabich.
The Mannheim orchestra introduced sudden crescendos known today as the Mannheim Crescendo which involved the entire ensemble swelling together. They also employed the Mannheim Rocket, a swiftly ascending passage featuring a rising arpeggiated melodic line combined with a crescendo, along with the Mannheim Roller, Grand Pause, Mannheim Sigh, Mannheim Birds, and Mannheim Climax.
Musicians from the Mannheim school developed compositions that influenced major symphonists like Joseph Haydn and Leopold Hofmann. Their collective work established a distinct artistic identity that spread across Europe through published scores and touring performances.
One chief innovation was establishing the four-movement symphony form with the menuet appearing as its third movement. This structure originally belonged to Baroque suites but gained new prominence here while the group played an important role in developing sonata form which became standard for classical symphony first movements.