Most photos are 4:3 or 3:2, so a 9:16 status either crops them or shows them with bars. If you crop, drag the preview to choose what stays. If you would rather keep the whole photo, WhatsApp adds a blurred background automatically when you post a photo that isn't 9:16; in that case still resize to 1080 px wide first so the visible part isn't recompressed twice.
Try it on your own photoDrop a photo at the top of the page. Ten seconds later it's sized, sharpened and ready to post.
Compress a photo