So, to make the process as basic as possible, picture or sketch a cape.
Most likely you began at the top and went downward.
A bottom-up (traditional) seamed sweater has several parts: front, back, sleeves and all are seamed (as well as the shoulders.)
The top-down raglans can be seamless and the entire sweater is made at the same time. The cape is made just like the top-down raglan until the shoulders.
The neckline is begun and stitches are added to increase. On the sketch of the cape I indicate two seams (plus are two seams in the same locations on the back).
When you drew or pictured the cape, your increasing came at the side; on a sweater the increases are at the "seams", one stitch on each side of four "seams". That is how the sweater gets larger. Note that they are not seams in the sense that they are stitched together as in a traditional bottom-up sweater; rather, stitches are there. If you have a raglan-sleeved sweater you can check this.
This process is continued until the underarms when the sweater is divided: front and back are joined and worked as one; plus two sleeves.
Soon I will be posting about my recently completed top-down raglan pullover and will show how a top-down is begun.