the Handle of a TBitmap is only a "utility" for the LCL to communicate with the Widgetset. For normal usage you don't need to worry about it (like every other handle)
If you want to draw something, create a TBitmap, set its Width/Height and start drawing using a canvas.
If you want to load an image, use LoadFrom... functions.
If you want a TBitmap to display on a form, put a TImage on a form, and assign its picture property.
If you want the RawImagedata, use a TLazIntfImage.GetRawImage
This is why I ask what you want to do with the handle.