A great deal of the "combat" footage taken during the Great War was actually staged, reenacted, or otherwise faked during the war. The same is true for a lot of the still photos of WWI "combat.". A lot of the photographs and footage of WWI battles aren't what they seem to be. Here are a few of their origin stories:.
Training films and photographs - This famous photo of an American soldier getting "gassed" was actually posed and used as a training aid to get soldiers to wear their gas masks.
Photographs of training exercises - If you see WWI photographs or film of troops charging towards the camera, it's very likely it was taken during training exercises.
Staged or reenacted - The famous scene in the Battle of the Somme of soldiers advancing across no-man's land and being "machine-gunned" is now widely-believed to have been staged.
Recycled movie footage - Sometimes, documentary filmmakers or YouTubers will try to pass off scenes from movies like All Quiet on the Western Front or grainier 1920s and 1930s movies as genuine combat footage.
Take the example of the famous 1916 documentary, the Battle of the Somme. The hand-cranked 35mm Moy & Bastie cameras were large, heavy, and had to be used on a tripod. There is quite a bit of genuine battle footage in the documentary -- however, much of the genuine footage is of events the filmmakers knew would happen ahead of time, like the famous sequence of the mine detonating under German lines. That gave them time to set up their cameras in a reasonably safe location and get a good angle of the action. This real footage is mixed in with staged footage of real troops acting out "combat" at the behest of the filmakers.
The same is true with "On the Firing Line With the Germans," which features some (almost certainly posed) battle footage of German soldiers. Interestingly enough, the movie was made by an American filmmaker prior to the U.S. entry into the war and is thought to be the only full-length wartime documentary made by an American.
Only in WWII, would there be cameras light enough to be hand-carried into battle with the troops.