I’ve mentioned before that I knit in the round because I hate stitching and seaming sweaters. But even a sweater knit in the round will need some finishing. You have some “live” stitches at least under each arm when you attach your sweater to the body, and those stitches need to be taken care of.
There are several options. You could knit and graft a gusset. I’ve never done this, but if you needed greater range of motion than a knit fabric will usually give you (I’m presuming you’ll be doing yoga in it or something, as knitted fabrics generally don’t bind much), or if you have very very large upper arm, attaching a gusset might not be a bad idea. (I sometimes do that in the crotch of pants because I do have heavy thighs).
Otherwise, you’ll just be grafting together your knitted pieces.
The first picture shows the live stitches. They’re not attached to anything. The second picture shows those live stitches attached by the Kitchener stitch. I mostly hate the stich because I’m not very good at it. I actually have to knit a couple of swatches and practice before I try it on a sweater I am making. That may sound a bit retentive, but I’ve put a long time into making a sweater, so I don’t mind a little refresher to try to make a structural seam work properly.
Knitting Help has a great video on how to do the Kitchener stitch.
And yes, I have to chant to myself, “Knit, slip, purl, purl, slip, knit,” as I am doing it. Stop laughing at me. I can’t help it.