An operator can be written in matrix form to map one basis vector to another.
You are just beginning to understand what basis vectors are.
Frequently, its not merely a single product, but a sum over such products involving all the basis vectors in the space.
A crystal system is described by three basis vectors.
A vector can be expressed in terms of basis vectors.
This then creates a set of orthogonal standard basis vectors.
However, in a general curvilinear system, there may well not be any natural global basis vectors.
For basis vector in the context of crystals, see crystal structure.
The general case is to give a matrix with the components of the new basis vectors in columns.
First, how can one estimate how many basis vectors are required?