The 1918 influenza was an upper respiratory virus (covid is a lower respiratory virus) that affected a disproportionate amount of young people.What killed a lot of people -- young people, too -- in the 1918 pandemic was an overactive immune response that was elicited by the virus. It just depends on the virus you're look at when you're consider immune system.
Most people died in 1918 from a secondary bacterial infection.
The big difference between now and 1918 is that they did not have antibiotics or the ppe we have today. They wore cloth masks and hand hygiene was not what it is today.