Technically, videos cannot be uploaded to Nostr, as relays only store json text notes, they don't store any media at all. There are media hosting services intended to be used with Nostr, but your Nostr notes still just have a text URL that points to where the media is stored. It is then up to each client to determine what they will do with that URL. Whether they will render the media that it points to directly within the client, render it as a URL that can be tapped or clicked on to open the media in a web browser, or render it as plain text. When it comes to videos specifically, how smoothly it will play often depends on how well the video has been optimized for web. Media hosts can help with that, performing optimization for videos that are uploaded to their hosting service, but not all of them do, so your mileage will vary considerably.