The billboard component is stored as the SpriteComponent.
Access
To get this component, you either subclass your root scene component, or from the owning actor you get the component by class using UBillboardComponent.
A single actor can hold multiple billboard components.
Creation
When UActorComponent::ExecuteRegisterEvents is ran, USceneComponent::CreateSpriteComponent will be called.
If bVisualizeComponentis true, the billboard component will be created, the sprite loaded and set and so on…
It seems that billboard components are usually created with an null sprite texture, so it uses the default sprite texture.
Some component light the light components uses SetSprite on the billboard component inside OnRegister (since the billboard component will be created at this point).