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I've read this in The Quill of Acceptance and the Book of Admittance on Pottermore, and the answers in: How and when did Hermione find out she's a witch?.
We know that a member of Hogwarts' staff goes and explains to the Muggle-born that they're wizards or witches. But how do they track them? For wizards families they simply send an owl, but for Muggle-borns, how exactly do they find a specific location that a Muggle-born resides.