The tiny dot is a so-called hyperkeratosis. The skin can be divided from the outside inwards into upper skin (epidermis), leather skin (dermis) and subcutaneous tissue (subcutis). In a corn, the outermost layer of the epidermis, the horny layer, known as the stratum corneum, thickens at one point.
The upper skin (epidermis) consists of several layers of irregularly shaped, flat, mosaic-like interlocked squamous epithelial cells, which are stably connected to one another by so-called tight junctions. A horny layer can form on the surface due to the death of horn-forming cells, the so-called keratinocytes, and the subsequent deposition of the cell bodies. The horny substance is keratin. Keratin gives the skin protection and stability.
In the case of a corn, the thickening of the horny layer is only noticeable as a dot on the surface of the skin. In reality, the spot you see is like a telegraph pole that you look at on top of it. That means, you only look at the circular end of a column that protrudes into the depth.