Battles don't occur in vacuo or in some abstract plane, they tend to occur when an offensive position meets a defensive position. Defensive positions tend to need to be spread out, to cover the entire front line. However, with sufficient forces an attacking army can concentrate offensive forces in an area without weakening the overall defensive line and yet still have a strong numerical advantage over defenders in that one area. This is a fundamental component of warfare, especially in the industrialized era.

To increase the odds of winning defending forces will naturally want to stack the odds in their favor. They'll use minefields to restrict where the enemy can travel. They'll build trenches and bunkers, they'll deploy anti-tank obstacles to make it more difficult for tanks to run amok or travel at high speeds (which also makes them easier targets for anti-tank weaponry including artillery). And, of course, defenders would naturally concentrate those defensive measures where they are most needed, near strategic objectives such as cities, etc.

In general, in many cases an offensive force will have to travel through an area that has prepared defenses in order to take control of new territory. For example, assaulting the Atlantic Wall during the Normandy landings in WWII. However, not all battles in the world wars were under those conditions.

In the case of the western front in WWI the front lines remained fairly static for a period of years so both sides had a very long time to build up fortifications and preparations. Trenches and bunkers were built behind the lines, and continuously improved over time. All out assaults across trench lines were rare, though sniping and artillery were omnipresent, so yes developments were made under fire, as it were. The "no man's land" between the trench lines was also developed as well, however. Strewn with barbed wire, for example. These things were often deployed under the cover of darkness but even so such assignments were extremely dangerous. Most defensive fortifications erected during the world wars were put up well behind the front lines in relatively peaceful conditions.