No app needed. Copy the post link, paste it below in Chrome and the MP4 is saved straight to your phone.
Open XGrokSaver in Chrome, Firefox or Samsung Internet. Paste the post link into the box and tap Download. When the list of qualities appears, tap Download next to the one you want. Your browser saves the MP4 and shows a notification when it's done.
If you started from a link inside the X app, it may open in X's own browser, which can block downloads. Tap the three-dot menu and choose Open in browser (Chrome) first.
In Chrome tap the three-dot menu and choose Add to Home screen. You get a one-tap shortcut that opens the downloader like an app, without installing anything.
Copy the post link in the X app, open XGrokSaver in Chrome, paste the link, tap Download and choose a quality. The MP4 is saved to your Downloads folder.
In the Downloads folder. Open Files > Downloads, or Chrome > Downloads. It usually also shows in your Gallery.
No. It works in the browser. You can add it to your home screen if you want quick access.
Yes. It runs in any modern Android browser, so the phone brand does not matter.