Learn Turkish: Ev büyük mü?

The question particle changes its vowel to match the last vowel of the word before it. This is called 4-way vowel harmony. Since the last vowel in büyük is ü, the question particle becomes mü.

Is "mü" attached to the word "büyük"?

No, the yes/no question particle is always written as a separate word. Even though it stands alone, it still follows vowel harmony rules based on the word right before it.

Where is the word for "is" in this sentence?

Turkish doesn't have a direct translation for the verb "to be" (am/is/are) for third-person subjects like "the house". Saying Ev büyük simply means "The house is big". Adding mü at the end turns that exact phrase into a question.

How do we know it translates to "the house" and not "a house"?

Turkish doesn't have a definite article like "the". The word ev can mean "house" or "the house" depending on the context. If you wanted to specify "a house", you would use the word bir (one/a) and ask Bir ev büyük mü?.