X re-encodes every video that is uploaded to it. The poster's original file is never stored and is not retrievable by anything, which means the quality ceiling for any post was fixed at the moment it was published. That is the single most important thing to understand about downloading video from the platform: a downloader cannot improve on what X kept, it can only avoid making it worse.
Where it gets uneven is that X applies different limits depending on the account. Premium subscribers can upload longer videos and at higher quality than standard accounts, so two posts that look similar in the timeline can hold very different renditions underneath. A clip from a standard account may top out well below 1080p even though the poster filmed it on a modern phone, simply because of where it was uploaded from.
Resolution alone is also a poor guide, which is why file size is worth reading. Bitrate — how much data describes each second of video — varies enormously between posts at the same resolution. A thirty-second 1080p clip stored at six megabytes and another at fifty are both 1080p, and the second holds roughly eight times the information. On fast-moving footage that difference is the gap between watchable and blocky.
The practical upshot: take the top rendition listed and check its size against its length. If 1080p is absent, X genuinely does not have one, and any site offering it for that post is upscaling — interpolating pixels from ones already present, which increases file size without adding a single new detail. We list the honest ceiling instead.