The MP4 option is worth choosing for watching offline, editing, re-uploading elsewhere, or archiving a post before it disappears.
Technically you are getting H.264 video with AAC audio. That is not a detail most people need, but it explains the compatibility: MP4 opens on every phone, computer, smart TV, console and video editor made in the last fifteen years. If a file needs to work on something old, unusual or not yours to configure, this is the format least likely to cause an argument with it.
This is a direct copy of the file X serves. Nothing is re-encoded, so the result is pixel-for-pixel what you would see on X itself.
The three options exist because they solve different problems. MP4 is the video itself, copied straight from X with nothing altered. MP3 is the sound on its own, converted so that anything with a speaker can play it. GIF covers the short silent loops X hosts, which are stored as MP4 whatever the button on X calls them. Picking the wrong one is not fatal, but picking the right one saves a conversion step later.
A minute of video at 1080p comes to roughly 10 to 15 MB, and at 720p about half that. A minute of audio at 192 kbps is about 1.4 MB, which is why people who only want the sound often take the audio option even when they could take the video.
Speed is limited by your connection rather than by any queue. Video downloads are a direct copy, so they finish as fast as your connection can pull them. Audio takes a few seconds longer because the sound track has to be separated from the video first, and MP3 adds a little more on top for the conversion itself.
Nothing here is upscaled, sharpened or enhanced. The quality list is read from the post, so if a video was published at 720p then 720p is the best that exists and no option above it will appear. Claims of 4K output from a 720p source are, without exception, an upscale: the extra pixels are invented, and the result is a larger file that looks no better.
One generation of loss is already baked in before you arrive, because X re-encodes everything on upload. That cannot be undone. What can be avoided is a second generation, which is why the video path copies rather than converts.
The process is identical on mobile. Copy the post link from the X app using the share button, open this page in your browser, and paste. On iPhone and iPad the file lands in the Files app under Downloads, from where you can move it into Photos. On Android it goes to the Downloads folder and usually shows up in Gallery or Google Photos shortly afterwards.
There is a faster route worth knowing: in the address bar of any X post, change x.com to downloadthisx.com and press enter. The download page opens with that post already read, which skips the copying entirely.
Saving a publicly posted video for your own use is ordinary and widely accepted. Watching something offline, keeping a copy of a post you made yourself, or quoting a clip under fair use or fair dealing are all normal uses.
What changes the picture is redistribution. The person who posted the video holds the copyright to it, and downloading does not transfer that. Re-uploading someone else’s work as your own is a copyright problem regardless of how the file was obtained. This tool only reads what is already public, and it keeps no copy of anything after the download finishes.