A: It starts with the plant material, then the extraction process. 
This goes for terpene extraction too! 

Clean extracts from reputable extractors use solvent-free methods or natural solvents, including food-grade ethanol, for initial plant extraction. 
Further proper processing of extracts is performed at lower temperatures, utilizing a gentler process and solvents that are entirely removed from the final product. This is the case for products like delta8 and delta10 THC distillates, CBN distillate, and more. 

Dirty or contaminated plant material and extracts can contain residual chemicals or heavy metals. 
Some processors run at higher temperatures to compensate for inefficient equipment, which when combined with heavy chemicals or contaminated plant material, can taint a product. 
This is why it is important to always do your research when trying new products.