Our editor provides three ways of adding videos in email/template.
1. You can insert YouTube or Vimeo link. To do this, add the video block to email...
... then click it and add the video link in the settings pane
2. You can insert custom video link using the Custom Thumbnail option in the video block
In this case, video must be allowed for all users by hosting, otherwise, they won't be able to watch it.
3. You can also embed your HTML5 video, it will playback right in the email.
Please note that in this case, not all email clients or browsers support video playback in email, some of them will redirect you to the video hosting page.
For more details on this type, read the second part of this post.