This guide covers how to use Transform.Find to find objects in the scene - including info on how to abuse its undocumented behaviour.
This can be very useful for Parenting Things to the Player Rig, as well as for other scenarios that require references to GameObjects inaccessible out of the editor.
Crawling up the Ancestry Tree
You can use the ../ syntax to traverse to the parent of an object.
The below gif showcases using this to parent an object (ParentToFloop) to another one that exists inside of the parent (Floop):
Crawling Down from the Scene Root
Similarly, appending / to the start of a path will base your path on the root of the scene.
This example showcases using / at the start to search the root, look inside of the Floop GameObject, then finally parent to Bar:
