I might be wrong but i don't think you can either import a video file into the project or preview it unless its on some kind of link so a way to drag and drop the video into a folder would be nice.
Doubt you're going to see that any time soon, we've asked for quite a while (years) and hopefully we're at least going to see a way to reference local files soon so we can utilize Relative links that point to files in a specific location (that will be the same setup when uploaded as well). One major reason why it hasn't been implemented is the size of the proprietary files would be huge for anyone having a lot of videos and audio files etc. Easier to just reference them outside of the project.
In the meantime while you wait, the best way to do this is to reference them with Absolute URL's if they are online uploaded somewhere. Once you have everything completed as you need, you can always update the links to Relative links if the videos or audios or PDF's etc. will be in the same directory tree as the website. I usually do that for testing to keep it simple. Once I know it works well, I many times don't bother with the testing and just move on to the next part, since I know it's ready to go.
You cannot import video (or other files) into Bootstrap Studio like you can import images. The program does not support this. To use a video on your website, you must upload the video to your server, and then reference the location in Bootstrap Studio with whatever method you're using to play the video(s) (ex. the HTML video component, custom code, a third-party player, etc.)
The reason is due to the nature of how the program saves the website (as a single compressed file.) Since videos can be many megabytes to many gigabytes in size, incorporating them into the saved file could make the file size potentially enormous, and take a very long time to both save and open the website.