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. at least Unity doesn't & 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
all of this is useful to know because unity doesn't support