>Where in the Bible does it explain the ritual of putting the black box on the forehead and wrapping … leather around your hand?
(Deut. 6:8), “You shall bind them as a sign upon your hand, and they shall be for a reminder between your eyes.”
The Torah mentions the mitzvah of tefillin four separate times. Each of these texts is inscribed on parchment and placed into the tefillin. These passages discuss the unity of G‑d. They describe the miracles G‑d performed for us when He took us out of Egypt, and how G‑d alone has the power and dominion to do whatever He wants in the physical and spiritual worlds. In other words, these verses cover the fundamentals of our faith.
https://www.chabad.org/library/article_cdo/aid/1918251/jewish/What-Are-Tefillin.htm