Short answers, pulled from the story.
German rearmament began in 1921 on a secret and informal basis, less than two years after the Treaty of Versailles was signed. The German Ministry of Defence established Special Section R that year to run clandestine armaments programs in cooperation with the Soviet Union.
Mefo bills were short-term treasury notes issued by the Metallurgische Forschungs-G.m.b.H., a shell company created by Reichsbank president Hjalmar Schacht. The company sold over 12 billion Reichsmarks worth of these instruments by 1938, with all proceeds channeled into the rearmament program while concealing the accumulation of government debt from the international community.
Around 150 American corporations took part in German rearmament. They supplied German companies with synthetic rubber technology, communication equipment, computing machines, aviation technology, fuel, military vehicles, and funding through a complex network of joint ventures and cross-ownership arrangements.
The Black Reichswehr was an extra-legal paramilitary formation created by Hans von Seeckt after the Freikorps were banned in May 1921. It was secretly integrated into the German military and had the backing of Chancellor Joseph Wirth. Its strength grew to an estimated 50,000 to 80,000 men before it was disbanded in late 1923 following the failed Küstrin Putsch.
Hitler publicly announced German rearmament on the 16th of March 1935, simultaneously reintroducing conscription. On the 26th of September 1938 at the Berlin Sportpalast, he boasted that Germany had rearmed to an extent the like of which the world has not yet seen.
Germany secretly trained pilots at the Lipetsk fighter-pilot school in the Soviet Union starting in 1924, using Junkers, Heinkel, and Dornier aircraft. A tank training school was established near Kazan in 1926 as part of the military cooperation program with the Soviet government.