What exactly is essence? The most popular is the Brand's Essence of Chicken, but there are also other products that are "essence", like "essence of fish" and even "essence of beef". There are also other terms that seem to mean the same thing like, "extract of something" and "essential oils".

What exactly are these essences and extracts? Where do they come from? How are they processed?

Edit: Chicken essence recipe
Based on the recipe you provided, it's kinda like concentrated broth. It's produced by cooking the chicken covered in a double boiler, effectively steaming it and collecting the drippings.

I have no idea if that's exactly what's being sold in those nutritional supplement bottles, but given that people think the homemade version is, well, the homemade version, it must at least be close.