A lot of mods add new items "to the tutorial chest", but none of them explain where that is. There are several chests on the Nautiloid - which one is the "tutorial chest"?
It's one of the chests you encounter on the Nautiloid during the Prologue, labled as Cartilaginous Chest.
It's dubbed the 'Tutorial Chest' since this part of the game acts as the tutorial, and mods add stuff to this particular chest because it's at the start of a new game and easily identifiable.
Source. Click for a more descriptive version, including the in-game map.
Note that the mod this screenshot is from allows for summoning the chest at any point in the game.