Resize an image to 1080 x 1920

Whatever you choose. Crop to fill scales until the 1080 x 1920 box is covered and trims what hangs over, so you get the exact size with the edges gone. Fit inside scales the whole picture until it fits, so nothing is lost and one side comes out shorter than the box. Stretch forces both numbers and changes the shape of everything in the picture. The default is crop to fill, because an exact size is usually the reason somebody came.

Because the shapes are opposite. A landscape photograph is wider than it is tall; this box is the reverse, and much taller in proportion than anything a camera produces horizontally. Filling it means keeping a narrow vertical strip out of the middle of your picture. For a landscape photograph, fit inside is almost always the better answer - or crop deliberately first, so you choose the strip rather than letting the centre decide.

Yes for crop to fill and stretch. This is the shape a phone already produces, so a portrait photograph from one is usually larger than the box and gets reduced rather than enlarged. A landscape photograph is the awkward case: filling this box from one means enlarging a narrow strip, which softens it noticeably.