My great grandfather was 18 in 1918. Some paperwork was saved by the family. It was an application to join the air force. But he clearly never sent it. The survival rate for pilots in WWI was scary. The training was next to none existent. For whatever reason he joined the army instead. We have his medals and he wrote a diary which wasn't permitted.
From the diary we know his location and which battles he was involved in.
Early in 1918 not long after he landed in France he was hospitalised with flu. This almost certainly would have been the first wave of Spanish flu. It had a devastating effect on young men his age in the main wave but the first wave was much milder. It probably gave him immunity so potentially saved his life later on having had that early exposure.
Being in hospital at the period he was, also meant he missed the last few offensives by the Germans which left the British with large numbers of casualties. In short, flu meant he missed the worst of 1918 on the front lines.
By the time he returned to the front line, the tide had changed and the German army was starting to collapse. He was involved in a few of the last battles of the war, but the number who died was significantly less than had been the case in the proceeding weeks he'd missed.
So there were three things that seemed to work in his favour in terms of luck. Luck many other men just didn't have.
So it depends on the depth of the information you can find out. Just knowing my great grandfather was a soldier in WWI isn't that interesting. Having the rest of the information and then matching that with army records and his diary makes it come alive and tells you stuff about subsequent generations
My grandfather, his son, would go on to enlist in WWII as a pilot and that was also his career after the war. His maternal grandfather had been a photographer in WWI teaching the pilots how to do air reconnaissance which was a totally new piece of military history so there was clearly a lot of influence from both sides of the family.