Short answers, pulled from the story.
The Einsatzstab Reichsleiter Rosenberg, or ERR, was a Nazi looting organization formed in 1940 under Alfred Rosenberg. Effectively controlled by Hermann Göring, it was tasked with seizing Jewish art collections in occupied France, Belgium, and the Netherlands, centralizing loot at the Jeu de Paume museum in Paris before shipping it to Germany. By the end of its operation it had seized 21,903 art objects from German-occupied countries.
The United States government estimated that Nazi forces seized or coerced the sale of approximately one fifth of all Western art then in existence, amounting to roughly a quarter of a million pieces. In Poland alone, over 516,000 individual art pieces were looted, representing an estimated 43 percent of the country's cultural heritage.
The Monuments, Fine Arts, and Archives program, known as the Monuments Men and Women, was an Allied effort to protect European cultural property and recover Nazi-looted art after the war. The Wiesbaden Collecting Point, directed from the summer of 1945 by Captain Walter Farmer, stored, identified, and restituted approximately 700,000 individual objects. The Allies discovered more than 1,050 repositories across Germany and Austria.
The Washington Conference on Nazi-looted assets of Holocaust victims was held from the 30th of November to the 3rd of December 1998, attended by more than 49 countries and 13 private entities. It was organized by the US Department of State and the US Holocaust Memorial Museum, and built on the earlier Nazi Gold Conference held in London in 1997. Following the conference, the Association of Art Museum Directors developed guidelines requiring museums to review their collections for provenance gaps during the period 1933 to 1945.
Hermann Göring assembled a personal collection of over 2,000 individual pieces, including more than 300 paintings, approximately 50 percent of which was confiscated enemy property. Art dealer Bruno Lohse served as Göring's adviser and ERR representative in Paris, staging twenty expositions of looted works specifically for Göring, from which Göring selected at least 594 pieces. The US National Archives Consolidated Interrogation Report No. 2 noted that Göring always managed to give at least the appearance of honesty through token payments or promises to confiscation authorities.
Well over 100,000 items looted by the Nazis have not been returned to their rightful owners. The majority of what is still missing consists of everyday objects such as china, crystal, and silver, though significant artworks also remain unaccounted for, including approximately seventy paintings from Paul Rosenberg's collection. Roughly twenty percent of the art in Europe was looted by the Nazis in total.