and what materials would be expended with auto-building trenches? I don't mean like a fully fortified complex with concrete pillboxes and command centers, i just mean 1000+ people with trench shovels digging away and creating a veritable maze or corridors. more or less exactly like WWI.
Beyond the wear and tear on shovels, there are materials that go into it. Most trenches beyond the most basic involve some kind of wooden shoring/cover, and once you're looking at WW1-style trenches there'll generally be underground safe points, with boarded reinforcing. Then you've got barbed wire and mines - so there would be a need for supplies to make even 'advanced dig-in' fortifications work at maximum efficiency. If you think about Kursk or El Alamein, neither were 'fortifications' in the HoI sense, but the Russian and Axis/Allied fortification belts would have required significant amounts of supplies to put in place.