I have crude idea that numerical diffusion arises while using upwind scheme and causes solution to deviate from its original one. But I am unable to understand how numerical diffusion phenomenon is (directly) related to advection phenomenon?

The numerical diffusion is not related to an equation specifically but is related to the way you discretize this equation.

Depending on the discretization stencil you choose (upwind, downwind, centered, hybrid...), you scheme can be diffusive, compressive or dispersive according to the sign and the order of the derivatives involved in the truncation error. You can understand that by deriving and analysing the truncation error through a Taylor expansion, I did an example here where I explain why the advection equation discretized with an upwind scheme produces numerical diffusion.

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