The secrete sauce is watering and location for the topiary frame. You want the watering to go up the inside when using moss, otherwise, moss will turn brown.
This is your standard frame (detailed) stuffed with moss. Lots of moss and no filler inside or live plants added. The frame was built in pieces.
The designer added a face mask for looks. This topiary resides in Palm Desert, CA. Water is scarce at this location and opted for succulents for less water to sustain life.
The entire design depends on the destination where the outdoor topiary will live.
Here is the best practice on topiary plants are below.