Common ratios and where they rule
4:3 is the classic camera and iPad ratio; 3:2 comes from 35mm film and most DSLRs; 16:9 owns video and widescreen displays; 1:1 squares dominate profile pictures; 9:16 vertical fills phone stories and reels. Platforms crop whatever you upload toward their preferred ratio, so pre-cropping keeps control over what survives.
Resize vs crop
Scaling both dimensions by the same factor preserves the ratio and the whole scene. Forcing new dimensions with a different ratio squashes or stretches everything in it. When the target ratio differs from the source, the professional move is crop first, then scale.