Piece By Piece
Where the real details come from are the 22 non-sex chromosomes, which are compared against very extensive libraries of information. How does this work? The algorithm will take an entire genome and receive little pieces.
For each piece that it takes, it will compare it to the reference data set. It will compare it to an entire list of probabilities, such as British and West African. This method is how you can understand the results of tests such as 23andMe.