How to find Equivalent resistance of solid regular bodies i-e solid sphere ,solid cube
Such problems are called "boundary value problems", and they are solved using calculus. The key issue is to figure out what the distribution of the electric field within the solid is, and how this affects the movement of charges (current distribution). Since these two quantities affect each other, it requires differential equations to describe the relationship. How you apply the external field determines the "boundary conditions".

Such problems can become very complicated, especially in 3D space, unless you can exploit geometric symmetries to reduce the number of degrees of freedom.