Armatures / Rigs, what are they & how do they work?

What is an armature made of?
An Armature / Rig is made up of bones.
Bones can be parented to other bones, which will make them inherit movement from their parent bone.


The order in which all of the different bones are parented to each other, is called the hierarchy.

What are bones?
Technically a bone is simply a point (pivot) in space.
These pivots have a name an orientation (rotation) & as previously mentioned a location.



What about scale?
scale isn't actually saved per bone in the armature, all bones have a default scale of 1, and have the ability to scale as soon as they're being posed. if you scale two bones in edit mode, you're just moving the points further apart.

this is also where some confusion begins due to the way blender handles / visualizes bones

But I thought bones were sticks?
well, no, not really. Unity doesn't treat them as sticks & most 3D file formats can't save bones as sticks.
Blender has bone tails, meaning that you can't have a bone that isn't connected to anything / doesn't have a child. Blender will create another bone at the end to make it a stick (this bone is called the tail, an end bone or a leaf bone).



When you extrude a new bone from the tail of another bone, it becomes a root bone with it's own tail
so basically as soon as a bone has a child of it's own, it doesn't have a tail bone anymore. (even if it's displayed that way)
This also allows you to change the orientation of a parent bone to not directly point at its first child.



End / Leaf bones / Bone tails
Blender will, by default, export leaf bones.
this means that it will create an extra bone where the bone tail is upon export.
I highly advise you don't export with leaf bones, as it can add useless bones that impact the performance of your avatar.
Additionally, if you import and export a handful of times with end bones on, you'll get a giant spaghetti chain of useless bones.



Using Leaf / End Bones for Dynamics
Sometimes you'll actually need end bones, Primarily for dynamics. Unity needs to know where a bone ends to put a collider there, or to know how much the bone needs to move (for ex. a long feather would wiggle more than a short one)
for this I'd recommend adding your own end bones wherever they're actually needed