In a nut shell the bacteria covert toxic ammonia to nitrate. A lot of people will use porous rock because it has more surface area for bacteria to colonize. Crushed coral is common because it slowly dissolves in the water and increases the hardness.
Lava rock is inert, so it won't effect the water at all. Its fine in a filter, but I would keep it out of the tank because it has sharp edges that can damage fishes mouths with they nip at things on the rock, or brush against it. Things are often added to the filter because that is where there is the greatest amount of water movement.
That being said, it isn't necessary to add anything. Bacteria will grow on any surface. In my opinion, the glass, gravel and any decorations should provide enough surface area to cycle an aquarium. If you need extra media, there are too many fish or they are being over fed.
The carbon, it can be changed out, but you would find that most people who have been keeping fish a while simply do with out it. Regular water changes are cheaper and better than carbon. Carbon does however make a good thing for bacteria to grow on because its so porous, in which case, its best to never change it.
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