Learn German: Wer hat den Wein gebracht?

It comes from the verb bringen (to bring). It is called "mixed" because it combines rules from both regular and irregular verbs! Like a regular verb, it gets a ge- at the beginning and a -t at the end. But like an irregular verb, the vowel in the middle changes from the "i" in bringen to an "a". Note that the consonants also change a bit, making it gebracht.

Why is it den Wein instead of der Wein?

Wein is a masculine noun. Because it is the direct object in this sentence (the thing that is being brought), it must be in the accusative case. For masculine nouns, the word for "the" changes from der to den in the accusative.

Why do we use hat right after Wer?

The question word Wer (who) is always treated as a third-person singular subject. Just like you would say er hat (he has) or sie hat (she has), you use the matching verb form for "who" and say Wer hat.

Why does gebracht go at the very end of the question?

This is the standard word order for the German past tense (the Perfekt). The helping verb (hat) takes the normal verb position right after the question word, and the actual action word, the past participle (gebracht), is sent all the way to the end of the sentence.