Secondary xylem and phloem in dicot stem are produced by

Vascular cambium forms a complete ring in the stem of a mature dicot plant.
The cambial ring becomes active and begins to cut off new cells, both towards the inner and the outer sides. The cells cut off towards pith, mature into secondary xylem and the cells cut off towards periphery mature into secondary phloem.
Apical meristems are responsible for primary growth in plants.
Phellogen (cork cambium) is a meristematic tissue which usually develops in the cortex region of a dicot stem. Phellogen cuts off cells on both sides. The outer cells differentiate into cork (phellem) while the inner cells differentiate into secondary cortex (phelloderm).
Axillary meristems produce axillary buds which are primary in origin.