How the crop is chosen
This scales your image up or down so it completely fills the target dimensions, then crops evenly from the center to hit the exact size required - the same "cover" behavior used by most design tools, rather than squashing or stretching your image to fit. If your subject sits off-center, crop the image closer to it first with the Image Cropper tool, then resize here.
Why the preset list is organized by platform rather than by raw pixel dimensions
Choosing "Instagram Story / Reel" is far easier to get right than remembering that specific size is 1080×1920 pixels - organizing presets by platform and use case (profile picture, cover photo, shared post image) means you never have to look up or calculate the correct dimensions for wherever the image is headed.
Why this always crops from the center rather than letting you choose a crop point
A center-crop is the safest default when the subject's position is unknown, and it keeps the tool simple and fast for the common case - but it does mean an off-center subject can get cut off, which is specifically why cropping closer to your subject first with a dedicated cropping tool before resizing here is the recommended order when your subject isn't centered.
Why this runs entirely in your browser instead of uploading your photo anywhere
The resize and crop are both done with the browser's own canvas drawing capability, which is powerful enough to handle this directly on your device - there's no technical need to send your photo to a server to resize it, which is why nothing is uploaded and the whole operation happens locally.