They Manufactured Their Own Weapons
One of the most remarkable things about the Polish Underground is that they were completely outgunned by the Germans. Many people joined the Resistance with nothing but a pistol, and they would not be outfitted with anything better. But the Polish Underground planned major offenses and would spend a year or more planning, creating, and stockpiling weapons and ammunition for a single offensive.
In 1939 the Germans took over the manufacturing facility of the Vis pistol, an exceptional firearm made by the Poles. But gunsmiths remained loyal to the Polish Resistance and continued making weapons for the Home Army out of their homes or basements, using whatever parts they could find or fabricating weapons themselves. Hundreds of these pistols were made in secret and passed to Resistance fighters. The guns may not have been as good as the Vis pistol but it was still a huge accomplishment.
The Home Army also created 700 Blyskawicas in underground factories in Warsaw. The sub-machine gun was modeled after German MP-40s and British-made Sten guns. They were designed and built by guerrilla forces in Poland and were made from parts that were screwed together and that required very little welding. One of the most brilliant aspects of the gun was that it was engineered to fire German 9mm pistol rounds.
One of the most ingenious ways that the Resistance forged ahead against an army that had more weapons and vehicles than them was to create their own armored vehicle. The Kubus was a civilian Chevy truck that was covered with steel and could carry 8 to 12 soldiers. The Kubus took only 13 days to build. It saw action in two failed assaults on Warsaw University. It sustained minor damage during the failed assaults and was pulled from use and abandoned just a few weeks after it was created.