I added a video to a post. Then I deleted this video from the post. I just want to make sure delete this video from the server too. Or when deleting it from the post did it delete it from the server too?
No, files still stay on the server when you delete them from a post. See also here, for example:
Ok, so is there a setting in ghost where I can set to automatically delete these unused files from the server? If not how where do I go to delete it?
No, you can’t do it automatically. If you’re self-hosting you could SSH into your server and delete them manually. Since you’re on PikaPods…honestly, no idea, sorry 
That’s bad news… Really there should be a file manager built into ghost, like wordpress has.
It’s a frequently requested feature. Make sure to upvote the one I linked above, for example.
Ok, just upvoted. Would you mind helping with my email issue on the other thread, please?