My understanding of WW1 had always been that its style of trench warfare was stable throughout the entire war. That is to say, the only defense against the new weaponry was to dig trenches, and the only way to capture trenches was to throw a few thousand men at them, at which point, you may have gained a few hundred yards. This continued until Germany ran out of resources.
What ultimately moved us past trench warfare, the story goes, was the improvement of the tank.
But a few years ago, I listened to a podcast (which may as well have been called Testosterone Drenched History, the actual name escapes me) that claimed that tactics had been updated near the end of the war; the war was becoming more mobile, ground was being transferred more quickly. It did not get into details.
I stumbled across this webpage (https://www.warhistoryonline.com/world-war-i/breaking-the-deadlock-machine-gun.html) that credits light machine gun units (with ample supplies of grenades) for breaking the stalemate.
And a piefed user supplied this somewhat cryptic (to someone who doesn´t read ~~German~~ Norweigan) diagram of how to take out a machine gun nest.
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[translation provided in comments: suppressive fire and toss in a grenade, repeat]
What is the true story of the tactical situation at the end of WW1?
To go a bit more into your overall question (and not into the diagram): No simple "it was because of X" answer will be worth its salt.
One thing that does come close is "better usage of machine guns". Before WW1 there was still Napoleon style warfare. The French army went in with blue uniforms with red pants instead of anything approaching camouflage, and the Germans with the famous pickelhaube, a helmet with a spike meant to deflect cavalry sabers.
Those armies with those cannons with the wooden wheels, at some point started having smaller cannons that had mechanisms to fire faster, getting closer to what we call a machine gun. But they were still seen as artillery pieces, managed at a higher level, ordered to a more permanent position. Enemy storms your position, if the make it you take the MG back to your second line, pick of those enemies who aren't dug in yet, and then go back to your first line. You do this with one MG and like, hundreds of guys with bolt action rifles.
In WW2 Germany started out with having an MG per infantry squad of 10 people - and that is still roundabout the ratio of modern armies. Because an MG, set up well somewhere, with just a couple of guys preventing easy flanking, is already a huge problem for any enemy who wants to traverse that area, even if they're a group of a hundred.
These were incredible strengths; quickly deploying it to make anybody in that direction be in cover or easy pickings, while also quickly REdeploying to make a situation uncomfortable. But it didn't just take a realization "oh, MG stronk!", it also took a development of production levels and tactics over years.
But, to segue back into the bigger point of there being no easy answer: All of that is very much connected to your industry. If the Germans would have wanted that ratio at the battle of the Somme, they would have needed roundabout 30000 MGs just for their soldiers who died there!
Then you look at other WW2 developments like the 1-person-carriable mortar. Which can hit people who are in cover, great synergy with an MG - but same thing here, it needs to be produced.
Submachine guns sure make storming a covered position like a trench a lot easier, but 1 of those among your 100 guys won't make a huge difference against 100 guys defending a trench.
There was a war going on, and A LOT of developments during that war were very much attempts to break that stalemate. Because that would basically be winning.
But there wasn't that one single trick that did it. It was more that everyone gradually got better and better at fighting it, building up a potential that they kinda couldn't fully realize because they were exhausted from years of fighting. So when they went into WW2 with that knowledge, but then powered by refreshed industrial capacity, the playing field was just noticably different.
Another perspective that might help understand is to not see trench warfare as a single thing. It got called "trench warfare" because the trenches were a striking feature, but it also consisted of a bunch of other little technologies and tactics. And most things that were developed for this trench warfare war, were still used in WW2. So, this perspective would be more to see it as a gradual change, where that one detail of having trenches at some point dropped out, just like a couple other things did at certain points. The shift from WW1 to WW2 was like shaking a tree so that a bunch of ripe fruit fell out earlier than they would have, had the war gone on.
Thanks, that seems like a mature way to look at it.
And the article I linked did specify that the German innovation was to concentrate the machine guns into special units instead of distributing them evenly, so you and the German high command are on the same page.