It is either a 4mm wide coverstitch, 4mm wide twin needle or 2 rows of single stitching.
This is a video for the 4mm wide twin needle.
My constructions seam is a wide seam allowance 4 thread serger seam. With better thread in the needles and regular serger cone or bulky nylon in the loopers. I have the cutting knife at the right most position. For my machine, a Singer Professional 5 (14T968DC) results in a 5.6mm wide seam allowances.
After using serger for the construction, I finger press the seam to one side and use an open toe embroidery foot, and along the fold of the seam with one of the inner edges of the presser foot. This gives a great way to align and get a very straight line of stitching.
For my compression leg sleeves, I don’t have to backtack or tie off the stitching, it will be serged over when attaching the upper thigh leg band and ankle cuffs.
This method allows for the greatest strength and durability of the seam, while keeping bulk and production speed in mind.
I have a few pairs that have been through the washing machine on gentle or delicate cycle, and in the dryer at low or medium heat. After over 100 washes the seam still look great, have held up very well. None have had any seam failures.
We use Woolite Dark as the lanudry soap for this and similar garments, have not had fading or color loss in the spandex/lycra sport fabrics.
Bonus footage at the end, this seam finish also helps reduce the left thread for showing if the seam is pulled open. I tend to try to match the left needle thread to the colors of the garment. With all seams with serger or sewing machine, handmade or retail garment factory produces garment, needle threads will show if you pull on the seam. We can not totally get rid of these issues but you can use methods and techniques to reduce the amount that shows.