Youtube has very good Help instructions for this - see : https://support.google.com/youtube/answer/6178631?hl=en
The most popular 3rd party stitching software are Video-Stitch or Kolor Autopano Video While Kolor will generate a stitch for you as a drag-n-drop solution, Video-Stitch will allow for more control in manipulating the image. See360° Video Stitching Software for more information
A 360 video is created with a camera system that simultaneously records all 360 degrees of a scene. Viewers can pan and rotate a 360 video's perspective to watch it from different angles.