What would you put in your second aid kit?
The hospital. First aid is really just short for First Responder Aid.
It includes things that a first responder would do to stabilize someone so they can reach an actual aid station, like a hospital or medical encampment.
We don’t call that second aid because that’s not how we think about prioritizing things.
First responder aid is the unique side of things that needs to be called out separately, as it always has a role to play, even in a hospital setting. The people who arrive first administer first responder aid until you can be triaged and seen at the appropriate time to reduce the greatest risk to the most people. Which may mean you go to the back of the line, or it may mean they move you up to the front in a triage situation.