Why the German trenches of the Great War were higher in quality compared to the British trenches?

For the most part, the Germans had much nicer trenches during the war, a reflection of several factors that came into play. The first was simply location. Having been the army on the offensive, although they obviously didn’t achieve their objective and win the war in those early months, when things their advance did grind to a halt and the frontlines began to solidify and turn into fortified positions, the Germans had much more lee-way in the choice of ground, leaving the Entente powers to simply get what they could, while the Germans were willing to cede a few kilometers here and there if it meant that they would be in the best position available.

Both sides, of course, created extensive trench systems, with the front line trenches facing each other, and then support trenches and reserve trenches laid out behind, connected with the communication trenches. Although the main system would usually cover ~150m from front trench to reserve, more further behind would often be constructed to, and this could easily stretch back for a mile or more, so as to provide ready made defensive positions in the event that the first lines were breached. In the supply and reserve trenches, both sides would be burrowing into the ground to create various shelters, and these could provide any number of functions, from sleeping to medical care to hiding from artillery. But while in the broad sense of things these ideas are comparable, they were, as seen in the film, quite unequal in execution.

What it comes down to is the comparative positions of the two sides. The Germans had occupied French and Belgian territory, and were determined to hold it. They were less concerned with forcing a great offensive at the time than they were keeping what they had already taken, with more of the offensive planning being seen in the East against Russia. Victory there would then free up troops to allow for the great, final blow to end the war on the Western Front. That isn’t to say they never made offensive actions – see, for instance, 2nd Ypres – but they were quite willing to put time, and effort, into the construction of complex defensive structures, pouring tons of concrete and digging deep to construct their bunkers. It took time to organize and design, but by 1916, the Germans had begun extensive use of the ‘concreting’ of their positions. The German field manual of 1916 described the general attitude thus:

Field positions when constructed afford considerable advantages to the defense. The important points to be borne in mind by the defense in a war of positions [includes]: utilization of ground so that conditions favorable for combat are obtained, while they are made unfavorable to the enemy.

On the other side though, sitting idle and twiddling thumbs was hardly the plan. The British commanders believe that they needed to be on the offensive. Pushing the Germans out was the goal, and any defensive line that they were occupying, ideally, were intended to be mere temporary accommodations. Why put all that time and effort into building into it if you are hoping to be moving forward soon? The result was the mismatch that is reflected in the film 1917, as you mention, with German trenches comparing quite favorably to the British ones in terms of the construction and accommodations, and a factor that the Tommies commented on when they had the chance to observe this. Although the front lines weren’t always that different, many German front-line trenches not being of concrete construction, and instead the duckboards little different than the British enjoyed, moving to the supply and reserve trenches, the differences would be impossible to notice.

Germans sheltered in deep, concrete bunkers, while they might be relegated to cover provided by corrugated metal and sandbags, if not merely a small nook carved out of the side of the trench. The bunkers often would be strung for electric lighting, and the various electrical and communication wires buried deep to protect them from shell-fire. Multiple British accounts remark on the presence in officers’ bunkers of glass “windows”, which wouldn’t actually look outside, being buried meters deep, but used a distorted, mirrored glass to at least give the illusion of it for the occupants. There were hardly aberrations, either, of some particularly important strongpoint of the Western Front, but reflected the German trench design and accommodations throughout the lines, providing their men both with a level of comfort, and defensive protection, that those facing them did not enjoy.

They worked too, of course! While we can’t boil things down to any single factor, we certainly can say that the construction and planning of the German trenches helped in ensuring that the Allies were unable to pull off a major breakthrough of the German lines in the middle period of the war. And even if the first line was taken of course, a maze of concrete bunkers and well constructed defensive positions would extend back extensively for a defense in depth. Eventually, the British began to put a bit more effort into their trenches too, it should be said, with concrete pillboxes prefabricated in Britain and then shipped across the channel for placement on the line, and introducing better engineered designs for the trenches themselves by 1917, but they never quite equaled the Germans.

And as for the Germans themselves, their lines constructed in 1915-16 were impressive enough in comparison, but those lessons were taken and applied to the vaunted Hindenburg Line (or Siegfried Stellung as the Germans called it), which was constructed in the early months of 1917, and, as plays a part in the plot of the film of that name, saw the Germans at points in the line execute a purposeful withdrawal to these new positions, ceding at some points a noticeable amount of territory, but with the expectation that it was a fair trade off, shortening the length needed to defend to better utilize manpower – it freed up 10 divisions – and maximize logistical capacity, and of course, to be ensconced in an ideally placed defensive position of superb construction that put to use the lessons of the previous two years, mostly abandoning the ‘front-support-reserve’ design to create a much more flexible defense in depth with a carefully designed network of positions.