Copy the individual post link
In X, open the post and choose Share → Copy link. Open this website in Safari, paste the link into the download tool, and select Get video. You do not need to sign in to X through this site.
Request the MP4 download
Check the preview and choose Download MP4. If Safari asks whether to download the file, confirm using Safari’s own prompt. Keep the page open while a large file starts transferring.
A download request is not a confirmation that the file has been saved. Network interruptions, storage limits, and browser settings can affect the result.
Find the file in Files
Tap Safari’s Downloads button to check progress or open the downloaded file. You can also open Files → Browse → Downloads. Depending on your Safari settings, the folder may be under iCloud Drive or On My iPhone.
Apple explains where to find downloads on iPhone and iPad.
If you want a copy in Photos
Open the downloaded MP4 in Files and check the Share menu for a save-video action. Availability depends on the file and your iOS version. The website cannot place a video directly into your Photos library without a supported device action.
If a video opens for playback instead, use the browser or device sharing controls available to you. Do not install an unknown app or provide your X password to save a file.