I wanted to know how does a segment get populated.
When I create a trait it starts getting populated with the audience that comes after its creation. Is it the same with a segment or a segment gets populated with the audience already present in a trait?
Segment and trait population both show the number of devices that qualify for it.
However, segment's population depends upon the Profile Merge rule selected.
If you selected a Profile merge rule as no authentication + current device, then it acts similar to a trait, and will show equal number of devices a trait has.
If you selected a profile merge rule that is current authenticated or last authenticated with any Device Graph option, then segment will show only those devices that will have a real time activity on your web properties.