Answer: These consist of several upright stems braided together as they grow. Leaving the upright top growing tip untouched, side branches and foliage are gradually removed to create the "tree" shape. When the tree has reached the desired height, the tip can be pruned to stimulate branching. It is a time consuming process and takes special care to achieve. Here is one explanation with photo you may find interesting.
I hope this helps.
Q&A Library Searching Tips