So do they thin out the flock as well when they bring in the new girls? If so, what does a person do with these hens?
Yes, that's the general approach (add and subtract) in order to keep flock numbers the same over time. So, you end up with a multi-age flock consisting of: young pullets from the current year's hatch, 1 and 2-year old hens, and 3+ year old hens. The older hens (3+ years) are culled in the fall when they begin to molt, making room for the younger pullets. The culled hens are used as stew hens to make soup. A few of the older hens may be retained in the flock rather than culled if they meet flock specific objectives (e.g., if they are good mothers, good layers, and/or needed for breeding).
This is the approach that I use, and it ensures that I have pullets coming into lay in late fall /early winter to provide eggs throughout the winter while the hens are taking a break to molt. With this approach, I've never had to use supplemental lighting to maintain the egg supply. It also keeps my flock numbers under control as I'm not adding to the flock unless I'm also prepared to subtract.