Trenches weren't built all at once. In most battles they started as small two man fox holes that were expanded over time into the trench networks we think of. The primary purpose of these foxholes, and later trenches was actually to prevent artillery spotting, because WW1 is foremostly a demonstration of the power of artillery. Western Generals at the start of the war had no idea of its full capability. Commanders were ill prepared for the new modes of war being made complacent by short colonial engagements where battles were decided in a day. The reality of WW1 was that these battles lasted weeks and sometimes months.

You are probably thinking of the elaborate trench networks of the western front when you talk about WW1 trenches so i can tell you how those came about specifically. WW1 actually started out as a very mobile war for the first few months which most people forget. However as the initial offensives on the west came to a halt trench war developed. It started with the Germans retreating from Paris. After failing to take the city and losing in battle they retreated back a plateau overlooking the Marne and began digging in. It was a very defensible position and so the germans used it as a way to halt the french advance so that the germans could retain the valuable industrialized region of France they occupied. Under heavy fire from the German artillery, the french also dug in, so they could prevent their troops from being slaughtered. Thats how we first developed two lines of opposing trenches in France. When one army tried to flank the other, they would extend their positions (and therefore trenches) to either side. However, if both sides are attempting this you just get elongated trenches. This is how "the march to the sea" started, where both sides tried to flank the each other and the result was a massive line of trench networks that extended from the North Sea to the Swiss alps, where water and mountains prevented the trenches from being further extended

Here's a great video about the week where trench warfare started in ww1 during the retreat to the Marne., it's from a great channel that covers WW1 week by week for the course of the war. Really great for truly understanding the war and i watch it whenever i have 10 min to spare. https://www.youtube.com/watch?v=hJV3ZGclI8o