The Vector3's class constants are Godot's global (world) directions. Vector3.UP, Vector3.FORWARD, Vector3.LEFT, etc.
Note that Unity's forward direction is the position Z-axis while Godot's forward direction is the negative Z-axis. The back directions are also inverted.
The local space direction of a 3D node is called Basis and it can be accessed with global_transform.basis.
- global_transform.basis.x is the local right direction.
- global_transform.basis.y is the local up direction.
- global_transform.basis.z is the local back direction.