Most videos uploaded in the past couple of years would be compressed as MPEG4 (h.264, specifically), but regardless of the video format the audio/video would be

Most videos uploaded in the past couple of years would be compressed as MPEG4 (h.264, specifically), but regardless of the video format the audio/video would be muxed into one file. If the video track consists of one still image, the impact (serverside compression, storage, delivery) of the video track would be minimal.