Sentence A is gramatically wrong. I imagine you mean something like: "Why are still working at building a car by yourself despite knowing it's very hard?"
As for sentence B, both "wonder" and "miracle" are usually associated with the verb "to work", or even "to perform" miracles.
Hmmm... Not exactly, although they are very close.
The literal meaning of miracle is an event which is not possible, but which happens anyway because a god or supernatural force makes it happen. If your legs were cut off, and Jesus (or Buddha or Krisha or Mohammad or the flying spaghetti monster) suddenly appeared and blessed you and then your legs were back on your body and fine, this would be a miracle. However, many people use the word "miracle" more loosely. If your leg was cut off in an accident, and surgeons worked for hours to reattach it, and a year later, after many months of healing and therapy you could walk on it again, you might say "It's a miracle!" It does seem miraculous, but your leg was reattached by science. Maybe you believe that God had a hand in your healing (and maybe you don't) but it isn't as though there is no explanation for how your leg got better.
People use the word "miracle" to mean something very unexpected and beneficial. This is a bit of hyperbole. But so many people use the word that way that this meaning is in the dictionary.
A wonder is something that is so impressive that people are filled with awe. If a surgeon could reattach a severed limb that would surely be a wonder.
So in a way, the two words are synonyms, but they actually have slightly different shades of meaning.