Resize a video to 2560x1440, the QHD frame between 1080p and 4K.
The bitrate tier argument
YouTube encodes each upload into a ladder and funds the higher rungs more generously. A clip delivered at 1440p is encoded in the 1440p tier, and the player serves that tier to anyone whose screen and connection can take it, downscaling as needed.
The result is that a 1440p upload can look better during 1080p playback than a 1080p upload does, because the bits behind the picture were allocated at the higher tier. This is why the size is popular with screen recordings and anything carrying text.
It is a real effect and a modest one. It does not rescue footage that was soft to begin with.
Scaling into it
3840x2160 divides by exactly 1.5 on both sides, so a 4K source scales down cleanly with no rounding.
1920x1080 scaled up to 2560x1440 is interpolation. It gains the bitrate tier and no detail, which is a trade some uploaders take deliberately and others consider not worth the file size. The tool warns when the target is larger than the source.
Outside YouTube
2560x1440 matches most 27-inch monitors, so it is a reasonable master for a screen recording meant to be watched at full size on a desktop rather than in a feed.