Generally speaking, charcoal is made out of burnt wood. Sounds too simple, but this is what basic charcoal comes from.
When you get into brands made in high volume, like for barbecue, I honestly do not know how they manufacture this stuff but guess it involves factory processes similar to various high volume output operations that include the packing together of various substances natural and non. Not sure about iron content, generally this is provided by leafy plant material still in a wholesome condition. Charcoal in the bowel does provide a scrubbing & absorbing action.