There are a few brands of t-shirts that don't have the side seam.
Hanes Beefy Tee, M & O Knits, Gildan 2000, American Apparel (I think), bamboo t-shirts, etc. There are probably others.
As to "how" it's done, I'm not really sure. Maybe one of our members with more experience with "cut and sew" might have some tips. I'm guessing it is sewn at the top and sleeves instead.
It's a tubular knit, the machine knits it like that. I've never really bothered to find out how because I couldn't see how the knowledge would be useful