You can add video's to the templates by clicking the small bar in the left bottom corner of an area or by dragging a video component into a content area.
After selecting the video area you can click the video icon in the toolbar and paste a link to a YouTube or Vimeo video. Both have free accounts to host your videos. Vimeo also has paid accounts if you want more customisation.
If you use YouTube you have to make sure the video is Public or Unlisted. When it's private other people won't be able to see it. If you use Vimeo you should make sure it's public or you should use their options to embed a private video but you will need a Plus, Pro or Business account for that.
We will use the thumbnail you set in YouTube. If you change the thumbnail make sure you change the url of the video one time to make sure it loads the new one.