Developer consists of small iron particles (dual component). The developer is mixed with augers churning all the time to keep the toner mixed evenly in the developer unit. The Magnetic roller (mag roller) holds the toner and evenly distributes the toner. Think of the developer as golf balls and inside the dimples of the golf ball the toner is carried (developer is sometimes called carrier) and evenly distributes it onto the drum or photoreceptor. When the developer material wears down the machine will dust with toner and cause backgrounding because it is unable to carry the toner as good as it did when it was new.
When changing the developer you must dump the old developer material out, vacuum and replace it with new material. You must always set the toner sensor before making copies and this takes knowing the simulation codes and how to set them. It’s really not that hard you will just need the information.