It's very complicated. Germany wanted a three way alliance between Germany, China, and Japan, starting with all parties joining the Anti-Comintern Pact, but Chiang Kai-Shek refused to join a pact with Japan while they held Chinese territory from the First Sino Japanese War.

The Nazis tried to modernize the KMT forces so they could chase Mao's tattered and at the time thoroughly spanked army into their mountain hideouts, but the KMT was, quite frankly, a clusterfuck of corrupt nepotism hires so it didn't really work out.

The KMT losing Nanjing pretty much immediately following the outbreak of war firmly ended any further cooperation, as Germany's goal was gaining access to resources and it seemed like Japan would get them all in short order anyways.

However, Chiang Kai-Shek and Mao, and most of their allied warlords, set aside their differences to try and repel the Japanese invasion, and rolled most of their armies into the NRA, which funnily enough was originally organized with Soviet aid in the 1920s to help defeat the leftover Qing warlords, and were able to stall out the Japanese in the interior.

It was still an absolute clusterfuck, and China in total lost over 22 million citizens to combat, famine, and outright genocide from the IJA, while inflicting 2.5+ million Japanese casualties (the majority of the losses Japan suffered in WW2 btw), but at least initially the KMT and Communist forces cooperated, with the KMT receiving aid from America and the Maoists receiving aid from the Soviets.