What the “p” number describes
Labels such as 360p, 720p, and 1080p refer to the vertical pixel count of the video frame. Higher resolutions contain more pixels and usually require more data to store and transfer.
360p and 480p
These lower resolutions are useful when bandwidth or storage is tight, or when the video will be watched on a small screen where extra detail is less noticeable.
720p
720p is a useful middle ground. It can look good on phones and smaller laptop windows without carrying as much data as higher-resolution video.
1080p
1080p is a common choice for larger displays and clearer detail. It usually requires more data than 720p and may involve separate video and audio streams that the server must merge.
1440p and 2160p / 4K
These settings can produce very large files. They are worth considering when the source truly offers the resolution, the display can show the detail, and you have enough bandwidth and storage.
The source is always the ceiling
Requesting 2160p from a source that only exists at 720p does not create real 4K. The downloader can select and combine available streams; it cannot invent image information absent from the source.