Short answers, pulled from the story.
The Free State of Prussia was established on the 9th of November 1918 following a mutiny by sailors of the High Seas Fleet at Kiel. This event sparked a revolution that swept across Germany and led to the abdication of Wilhelm II as Emperor of Germany and King of Prussia.
Most German territorial losses required by the Treaty of Versailles affected Prussia directly, with Eupen-Malmedy going to Belgium and Danzig becoming a free city under the League of Nations. Large areas of the provinces of Posen and West Prussia became part of Poland, leaving East Prussia separated from the rest of Germany by the Polish Corridor.
Otto Braun replaced Paul Hirsch as Minister President following the Kapp Putsch in March 1920 and formed the Braun-Severing system which became synonymous with democratic Prussia. The Social Democratic Party provided the minister president for almost the entire duration of the state's existence until its formal end in 1947.
The Free State of Prussia was legally abolished on the 25th of February 1947 by decree of the Allied Control Council. This event marked the final dissolution of the constituent state which had existed from 1918 to 1947.
Prussia continued to be the dominant state during the Weimar period because it was home to the federal capital Berlin and held roughly three-fifths of Germany's territory and population. It was considered a bulwark of democracy within the Weimar Republic due to governance by pro-democratic parties and political stability compared to the Republic itself.