what is the ingredient that makes my takeout lo mein smoky tasting?
If I'm understanding your question right, you might be referring to "wok hei," or the "breath of the wok." It's the flavor that restaurant food has because of the high heat of the woks in a professional cooking environment, and home stoves have a very hard time reproducing it. I found it covered in another Cooking Stack Exchange here:

What Is Wok Hai And How Do I Get It In My Food?