A TARDIS is not built it is grown.
The process used to create a TARDIS is one of the greatest secrets of the Time Lords.
The initial block transfer equations necessary to create a Complex Space-Time Event like a TARDIS
can only be assembled in a null-zone connected to the Eye of Harmony. This null-zone is called the Hyper-Loom or sometimes a womb. In a very real sense, Complex Space-Time Events like a TARDIS always exist.
These Events take the form of a complicated coded-series of Block-Transfer equations, which define their existence and position in Space-Time. The equations are based on the fundamental principles of movement, travel
and infinite recursion.
They serve as the foundation for the Time Travel Capsule (TTC).
Being a four dimensional being, a TARDIS assists the Time Lords in its own creation.
The Hyper-Looms provide
the Capsule with a Block-Transfer connection to the Prime Eye of Harmony on Gallifrey.
It is this Block-Transfer connection that serves as the heart of the Complex Event. The Space-Time Event combined with the Eye of Harmony connection forms an Extra-Dimensional Framework known as the Metastructure.