So let’s say British and German troops were fighting, would they be like, “How dude let me build my trench before we murder each other.” Like how did that work?
The front lines, at least initially, evolved organically where two armies came into contact and neither could rout the other. Individual soldiers would seek shelter in any depressions they could find shell holes, drainage ditches, furrows, simple dips in an otherwise flat landscape. When soldiers did have shovels (not all did when war broke out) they could dig themselves a foxhole and seek shelter in that. As battles wore on and the weapons of modern war made it ever more dangerous to move around above ground level, soldiers would expand their holes in the ground, connecting them to their buddies a few meters away. Given enough time, what started as a simple hole in the ground eventually expanded into an interconnected network of earthworks and trenches that soldiers could move freely through with a reduced risk that they might be killed by artillery or machine gun fire.