https://europecentenary.eu/a-german-russian-secret-agreement-the-treaty-of-rapallo/
At first, nothing seemed wrong with this amicable agreement. But behind it, there was a secret arrangement that would allow Germany to break the obligations of Versailles. It would be Germany’s first step towards remilitarization, and it would end with a conflict even bloodier than the one that had just ended.
….
According to the secret agreements, the USSR would allow Germany to train her soldiers on soviet ground. In return, Germany would send to Russia officers to train the new Red Army. At first, Germany seemed to gain more from this arrangement.
https://www.uni-wuppertal.de/en/transfer/science-communication/jahr100wissen-/-100-years-ago/100-years-ago-in-2022/the-treaty-of-rapallo/
Mangold-Will: Contrary to contemporary rumors, the treaty did not contain a secret additional protocol on military cooperation. Rather, as I said, this was negotiated in advance and in the surrounding area. Consequently, the economic cooperation agreed in the treaty mainly concerned activities that de facto circumvented the military restrictions of the Versailles Treaty. A well-known example is the construction of an aircraft factory by Junkers-Werke and the establishment of a flight school in Russia, where not only aircraft were built with German assistance, but German and Russian pilots were also trained for military use. Thus, in practice, the buildup of a German air force, which was expressly forbidden by the Treaty of Versailles, took place in Russia between 1925 and 1933.
So tank, aircraft, chemical weapons etc manufactured in Russia for Germany and trained on by both in Russia in order to get around Versailles restrictions on German military.