Crop a video to 16:9, the landscape frame for YouTube, televisions, and laptop screens. 1920x1080 and 1280x720 are the same shape at different resolutions.
Going the other way
Most work at this ratio is the reverse of the usual job: a vertical phone recording that has to become a landscape upload.
Cropping is rarely the answer. A 1080x1920 clip cropped to 16:9 keeps a 1080x608 band, which is 32% of the height, and the band almost never lands on the part of the subject you want.
Fit with bars is the standard fix. The full 1080x1920 picture sits 608x1080 in the middle of a 1920x1080 frame, with 656 px of background filling each side. Bars at the sides are pillarboxing rather than letterboxing, but the control is the same.
A blurred copy of the frame is what most channels use for this. It keeps the eye on the centre and avoids two hard black slabs.
When the source is already wide
A 21:9 or 2.35:1 clip cropped to 16:9 keeps the full height and trims the sides, which is a light cut: a 1920x822 ultrawide frame becomes 1462x822. Check that nothing composed against the left or right edge disappears.