Downloads on Chromebook go to the Downloads folder in the Files app. Open the Files app from the launcher and click Downloads. On a Chromebook this folder lives in local storage, so it does not sync to Drive unless you move it there.
The file arrives as a standard MP4 with H.264 video and AAC audio, which is the same container X serves to its own apps. Nothing is renamed, wrapped or converted along the way, so the file you get is byte-for-byte what X published. That matters if you plan to edit it later: a re-encoded copy loses detail every time it is processed, and a direct copy does not.
The quality list is built from the post itself, not from a fixed menu. If X only published a clip at 720p, that is the highest option you will see, because there is nothing above it to offer. Upscaling a 720p video to 1080p adds file size and no detail, so it is not something this tool does.
On a Chromebook, 1080p is worth taking whenever it is offered and you intend to watch on a large screen or edit the clip. 720p halves the file size and is hard to tell apart on a phone-sized display. 480p exists for slow connections and for archiving a lot of clips without filling your storage. As a rough guide, a minute of 1080p runs 10 to 15 MB, 720p about half that, and 480p around 2 to 3 MB.
Switch to the audio tab and you get the sound on its own, in two forms. M4A is the original AAC track lifted straight out of the video with nothing re-encoded, so it is an exact copy of what was published. MP3 is a real conversion at 192 kbps, offered because some older players and car stereos will not open an M4A file.
On Chromebook, both formats play in the built-in ChromeOS media player and any Android video app. Take the M4A if you have any choice, since it is smaller than the MP3 at equal or better quality. Take the MP3 if the file needs to open on something with no idea what an M4A is.
If your Chromebook is managed by a school or workplace, downloads may be restricted by policy. Nothing here can work around that, and you should not try to: the restriction is on the device, not on the website.
If the post itself will not load, the usual cause is the account rather than the device. Protected accounts, deleted posts and posts restricted by age or region cannot be read, because the video is not publicly available to fetch in the first place. Copying the link again from the post page, rather than from a quote or a notification, resolves most of the remaining cases.
Once the download finishes, the file opens in the built-in ChromeOS media player and any Android video app without any conversion step. Because it is a plain MP4 with no DRM, every editor on ChromeOS accepts it directly on a timeline.
There is one thing worth knowing before you edit: X re-encodes everything uploaded to it, so the version you download is already one generation away from whatever the poster originally shot. That is unavoidable and true of every downloader. What you can avoid is adding a second generation of loss, which is why the file here is copied rather than converted.
Anything publicly visible on X can be read: a post is either public or it is not, and this tool only ever sees the public version. Protected accounts, direct messages and anything behind a login stay out of reach, by design.
Downloading a video does not give you the right to republish it. The person who made it still holds the copyright, exactly as they would anywhere else. Saving a clip to watch offline, to keep a copy of something you posted, or to quote under fair use is ordinary. Re-uploading someone else’s work as your own is not, and no tool changes that.