Creating a Phylogenetic tree using DNA sequences
There is no single mathematical measure of relatedness.

In the context of phylogeny, you need a model of evolution in order to be able to evaluate some relatedness between your strings of nucleotides. Then you would have various option for how to build a tree so that it reflects this relatedness. See for instance the wikipedia page about computational phylogenetics: https://en.wikipedia.org/wiki/Computational_phylogenetics.

Through what kind of historical process were your sequences generated ?
If there is no historical process underlying the generation of your sequences, then it is not meaningful to build a phylogeny with them. You can still define distances between them and use clustering techniques to build a tree, but that tree will not be a phylogeny.