来たの??
どこから来たの？

I dont understand why it's "来たの" and not "来た".
Can someone help me?

Question

来たの??
どこから来たの？

I dont understand why it's "来たの" and not "来た".
Can someone help me?

Answer

The difference between 来た？ and 来たの？ is minor if you are asking a question.
来たの？ is a more explicit and casual way of asking whereas just saying 来た？ might be confusing whether you are asking  or not unless you pronounce clearly. (and could be arrogant.)
I would prefer to use 来たの？ imho :)