Why does it say kamu and not anda?
"anda" is formal and "kamu" is informal and it seems that duolingo is teaching the informal bahasa indonesian.
I guess Duo mixes it
No problems you say "kamu" or "anda". But, "anda" I think is too formally. "Kamu" is often to use.
"hello, you are from where?" could be another solution?
I agree! Why not? That is the literal translation and it is something that works in English too. Not the most common way to say that but it is accepted.
We can't translate it each word, and combine them.
That's gramatically wrong. The proper translation actually is "Where do yo come from?"
So are "dari mana kamu?" and "kamu dari mana?", both doable and acceptable like that?
Yes. But remember this is informal.
'Kamu dari mana' is usually used when asking where were they for a short time.
But we usually use 'kamu berasal dari mana?' when we mean to ask where are you originated from
Would Indonesian people really say that? It sounds like you're just walking up to a random stranger and asking where they come from.
Yes we do when we meet someone for the first time. Maybe it's uncommon in your country, but it's in Indonesia.
Why not: where are you come from?
To be (is/am/are) is only used besides adjective or in continuous tense. Example:
"Where are you coming from?"
Actually the proper translation is "Where do you come from?"