how do you go from knitting in the round to joining both sides of the circle into a flat piece

My friend has bought a beautiful scarf which consists of a hemline (which i think is made using Jane’s magic loop cast on) that goes all around the scarf. In the center there is a pattern which i think is the brioche rib. Now the hemline down the bottom and the sides can be separated, but the center can’t. I want to make one myself but can’t seem to grasp the technique - can anyone help??
Here are some pictures i took - hope it makes sense

Welcome!
It looks as though this is knit flat from one end to the other. There is a stockinette hem at each end and a stockinette edge on each side.
The hems can be made by working stockinette with a purl turning row and then either picking up sts from the cast on or using a provisional cast on.



You would have to work out how to do the side seams, probably by casting on and folding the edges over at the finishing steps.
This is likely not a hand knit so there may be machine techniques which are difficult to duplicate.

Thanks for your reply! Just saw the video that you linked and by looking at the scarf I can neither see nor feel any cast on the hemline - it’s as if it goes straight from hemline to brioche and the continues that way, until the end (as in the second pic in the album). And yeah I suspect that it’s machine knit too, but my stubbornness won’t let me give up.

The cast on would be part of the hem, where the body of the scarf joins the first row. The lower (and upper edges) would be the turning row.
Yes, the machine knit may conceal or get around this join.