Each pair of Tefillin is hand-crafted and made with tremendous attention to detail.
The pair starts with the hides, which are soaked, treated, and pressed. The batim (boxes) are formed from a single piece of leather, which is then squared, painted, and oiled. Then a sofer (Jewish scribe) writes four parshiyos (sections of the Torah) on parchment, letter by letter, ensuring maximum precision. Each letter has to be perfect. If even one letter is written incorrectly, the entire pair of Tefillin is not kosher.
Upon completion, the parchment and the writing get checked multiple times, both by a human eye and with state of the art technology, to ensure there are no errors. The parshiyos are rolled, tied, and set in their place in their respective compartments in the Tefillin boxes. The boxes are then sewn shut and repainted. The retzuos (straps) are then dyed black and cut by hand.
Each pair of Tefillin takes months of work that is done by multiple highly trained and halachicly knowledgable professionals in various locations, sometimes even in different countries.