For explaining the operation of the transformer Low voltage winding (LV winding) and High voltage winding (HV winding) are wound on different limbs of the transformer for easy explanation. However in practical each limbs of the core will have both LV winding and HV winding wound together.
In Transformer design LV winding will be wound close to the core of the transformer because the insulation provided between the LV winding and core of the transformer is quite less compared to the insulation provided between the HV winding and the core of the transformer which results in less cost for insulation and reduction in the size of the transformer for same MVA rating. Hence due to this reason LV winding is placed near to the core of the transformer.
On the core of the transformer insulation is placed and LV winding is wound. Once again insulation is placed between the LV winding and HV winding.