Find the Insert tab on the PowerPoint ribbon and click the Video drop-down arrow. You can add an online video from YouTube or another website, or upload a file from your computer. To insert an MP4 video from your computer, choose Video on My PC. After you've selected a video and told us about it and we showed you video thumbnail and the download button below it, the final step is to download video to mp4 file. For that, on desktop you can simply right-click the download button, and in the menu select Save Link as option.
A HTML video is likely a MP4 video or a Webm video in a specific format that is used to stream in the browser. Some of these can be saved directly, others have to be downloaded with ffmpeg and converted.