I'm not exactly sure which video you watched but I do know about using different size needles. When you knit in the round and don't turn your work the left needle is always the left needle, the right needle is always the right needle. Using a smaller needle on the left serves more than one purpose: Interchangeable sets will have 2 of any size needle and if you want to do 2 needle circular knitting, you don't have to buy a new set of needles, just use a smaller one on the left; if the needles are different it's easier to keep track of which one you should be using. All the left needle does is hold the stitches and present them for knitting with the right needle. I will use a smaller needle on the left for doing magic loop simply because it makes working the stitches faster and easier. I do magic loop because I will invariably knit with the wrong needle if I'm doing 2 needle circular knitting and make a mess. Others prefer the 2 needle method and don't have that problem. Clear as mud?
You can't knit the left sock first. Ever.