One upload, both halves
The separation service always computes both parts of the track: the voice and everything else. ToolGPT shows you the one you asked for and keeps the other on the result card. If you meant the opposite — you wanted the singing, not the karaoke version — flip the toggle and it switches straight away, without sending the file again.
For a video, the chosen audio is put back under the original picture, so you get a video with the vocals (or the music) removed.