is there rly a InventorySlotArgument.java? isn't it just an int?

Technically yes, it later remaps the slot to an int. But in the command it displays with fancy strings like hotbar.1, inventory.25, armor.chest, enderchest.12, weapon.offhand, etc for players.

There is also names for containers, like container.37, for villagers, like villager.6, and then for horses there is horse.saddle, horse.armor and horse.chest.

Yarn name: ItemSlotArgumentType.

yes, I want it to return type of entity/block with a valid slot to use with inventory api.
this makes so that all the vanilla containers/inventories supported by it should be supported by inventory_* in the API