The Vardy were extremely intelligent and diverse microbots capable of learning new skills and adapting to the tasks required, such as pollination of large fields of wheat. They moved by floating through the air in large swarms and could move very fast.
Vardies could interlock in order to create complex structures as large as buildings and machines but could easily break apart into swarms and attack anything they perceived as a threat. They could mimic the colour and texture of various materials such as glass, ceramics, plastic, wood and metals.
They killed their target by enveloping them at high speed and stripping them of clothing, organs and muscles, all within mere seconds. This form of attack made them similar to the Vashta Nerada. (TV: Smile)
On one human colony, the Vardies would gather together to make buildings as well as distribute fertiliser across the colony's wheat. By the time of the Twelfth Doctor's arrival with Bill Potts, the Vardy were a lifeform of their own, a species with sentience and awareness.
When the Emojibots first encountered grief, they considered it an enemy of happiness and thought of destroying it, they called upon the Vardy to kill anyone who wasn't happy, stripping them of skin, organs and muscle leaving behind their skeleton, which later would be used to make calcium-based fertiliser.
Before a massacre of grieving colonists could occur, the Doctor reset their memories and opted to open diplomatic relationships between them and the humans as an apology for his initial ignorance of their being a sentient life form. (TV: Smile)