What 4K, 1080p and 720p actually mean when you save a video
Resolution is only half the story. Bitrate is what decides whether a saved clip looks crisp or muddy, and the two get confused constantly.
Resolution is a pixel count, not a quality score
1080p means the picture is 1080 rows of pixels tall. 2160p, marketed as 4K, is twice that in each direction, so four times the pixels. That is all the number says. It describes the size of the grid, not how much detail was preserved inside it.
This is why a 4K file can look worse than a well made 1080p one. If the source was filmed on a phone in poor light and then compressed hard on upload, blowing the grid up to 4K only makes the mush larger.
Bitrate is the part that decides sharpness
Bitrate is how much data is spent on each second of video. Spend more and fast motion stays clean. Spend less and you get blocky shapes around movement, smeared grass and banded skies.
Social platforms re-encode everything you upload to keep their storage bill down, so the file you can save is always the platform's copy rather than the creator's master. That is a ceiling no downloader can lift.
Why some tools stop at 720p
Above 720p, several platforms store the picture and the sound as separate streams. A tool that only knows how to grab a single file has to fall back to the last combined stream, which is usually 720p.
Handling the split properly means fetching both streams and joining them, which is what the YouTube downloader and every other platform downloader here do before the file reaches you. When you see a downloader that never offers anything above 720p, that is the corner it cut.
How to choose in practice
Watching on a phone: 720p is genuinely enough, and a pass through the video compressor shrinks it further without a visible change.
Editing or reposting: take the highest available, because every later export loses a little more.
Archiving a talk or a lecture: the audio only track is often the honest choice, since nothing visual is lost that you will miss.