Hi, Sina, nice to meet you!
The metal end is mouthpiece. Persian playing technique differs from Turkish or Arab: You hold end of flute with your teeth and blow over your tongue in the reed.
Canot tell you more on this. All sources, what I know, refer to the Turkish/Arab style playing over the edge.
When you have no assistance and you would try learning by doing, I'd recommend you to start with the shortest flute possible, and continue with next size, after you mastered first.
Another philosophy is to begin with largest and master deepest tones first. Therafter, you would have no problem with any Ney. But I'm afraid such a method you cannot provide without assistance of an experienced player, who may show you a lot of those "hows", you are not thinking about. This method is similar to Yoga, it need some time to feel effection.
