Starfleet personnel are a subset of the total human population. Off the top of my head I'm not aware of any canon figure about what percentage of the human population is in Starfleet, or the total number of Starfleet, but presumably it is not a huge percentage of the total.

There are numerous notable civilian characters in Trek, but in general the franchise presents us with a vision where, contrary to the in-universe reality, civilian life is sort of at the periphery of Starfleet. Civilians are people that Starfleet personnel find to be colorful characters, sometimes become romantically involved with, people who serve Starfleet personnel when they're on vacation or otherwise at leisure, sometimes criminals, etc.

The notable civilian characters we see on screen tend to have their entire lives circumscribed by Starfleet – think Jake Sisko or Keiko O'Brien.

I'd be really curious to see not just a human civilian character but a civilian world fully developed over the course of a series. What is it like to be "just some person" living their life?