The back of a knit st shows as a purl, the back of a purl st shows as a knit. When you knit in the round (a tube) you only knit on the RS, so you knit every round to get stockinette where all the knit sts are on one side, and all the purls on the other side. To make garter stitch, you have to knit one round and purl another round.
The point of magic loop is to knit something that’s small, like socks or mitts or a sleeve without using dpns. You use a longer circular and pull out the cord between sts to shorten it up. They do make short circs 9 and 12 inches, but many people find them hard to work on, they’re not readily available and for some things, even they may be too long. You can also use 2 circulars to knit small things with, they work kind of like flexible dpns.
Since you can also use circular needles to knit flat - you just turn your work at the end of a row, like you do with straight needles - they’re very versatile and many knitters don’t even use straight ones any more.