Typically, there is a box (gutter box, water catch box) when connecting the downpipe from the flat roof (google image search "Fallrohr Flachdach"). I am missing that here and would appreciate technical clarification! (I know, this is a picture thread ... :rolleyes)
The drainage is, as probably expected, in the roof, and the drainage runs from the facade out the back into the downpipe. The outward appearance is simply elongated. The top 20cm of the downpipe are essentially non-functional.