where did u get idea that susanoo is fire release? or any release of that matter, it isn't even chakra nearly summoned with chakra.. it's one of legendary spirit items that aren't even up to speculate. it is a shield summon with only physical shape to see
Ameterasu is not an item.
If it was, Edo Madara couldn't use Sasanoo'O, which requires Ameterasu in order to be able to manifest Susanoo'O, because Sasuke would be the person with that item.
The Yata Mirror and the Totsuka Sword are items, and is specifically talked about in the Itachi vs Sasuke fight.
The simple fact that Ameterasu is existing in two places at once without being manufactured twice means Ameterasu is not an item.
If it's not an item, it's a release of some sort (not every technique necessarily has an element), but the fact that it directly effects the real world implies it's an elemental release of some sort.
Further supporting evidence of this is the fact that Ameterasu is repeatedly refered to as a type of fire technique from the time we first see it used against Jiraiya's toadbelly tech.
Further evidence is the fact that Sasuke quickly masters Blaze release after mastering Susanoo'O, implying that Blaze release is related to Ameterasu.
Susanoo'O is a fire tech, specifically Blaze Release, that uses Tsukiyomi to shape the Blaze release, in such a way as to allow the person in the middle of it to continue to exist without getting burned and to allow the blaze release to affect the real world.