These are all the spore card interactions I believe.
If you use spore cards for your actions (from hand) they go to your spore deck.
If they are discarded normally, they go to the shared discard pile.
If they are added because of necropolis interaction they go to the shared discard from the spore deck.
If they are removed via the necropolis interaction they are taken from the shared discard and put into the spore deck.
The Spore deck itself has a spot on your board that only spore cards can be added/removed to/from.
Hope this clears it up.