Resize a video to 1920x1080, the Full HD frame most players, editors, and upload forms assume.
Downscaling from 4K
This is the common case and the one that goes well. 3840x2160 divides by exactly 2 on both sides, so every output pixel averages four source pixels. That averaging suppresses sensor noise and mild focus error, which is why a 4K clip delivered at 1080p often looks cleaner than footage shot at 1080p.
The pixel count drops to a quarter, and the file with it.
Upscaling into it
Going from 1280x720 to 1920x1080 multiplies the pixel count by 2.25 and adds no detail. The file grows, compression artifacts in the source grow with it, and nothing looks sharper.
Do it when a platform or a client rejects anything under 1080p. Do not do it to improve the footage.
Vertical sources
A 1080x1920 phone clip is the opposite shape. Covering 1920x1080 keeps a 1080x608 band across the middle and cuts 656 pixels from the top and bottom, which usually removes heads and feet.
Padding is the normal answer, and it lives in the video cropper: the upright picture sits in the middle of the 16:9 frame at 608x1080, with 656 pixels of background on each side. A blurred copy of the frame fills that space better than black on a wide screen.