The other answers and comments are excellent.
Adding a video on how rims are commonly made and a picture of the seam.
A double wall aluminum rim starts life as a straight piece of extruded aluminum.(Factory Tour: Velocity USA Bicycle Wheels)
The bar is bent to shape (about 3:51 in the video)and cut to length (4:13). The ends are joined together with a sleeve and epoxy (5:20)
Sleeved Joint Connection
As Ralf points out since this seam is part of the manufacturing process both rims will have the seam.
If the rim maker does a good job the seam will be difficult to find. A variety of factors can make the seam more visible on one rim than another including manufacturing quality control and the type of stresses a rim experiences while riding. Knowing where to look makes finding the seam easier. As Michael says the seam is usually opposite the valve hole exactly between two spokes.