In more detail
Cropping keeps a rectangle and discards the rest, often to a fixed Aspect ratio. Rotating by 90° swaps width and height; other angles need resampling and leave empty corners to fill or crop. Flipping mirrors the image horizontally or vertically. Many phone photos are not actually rotated: they carry an EXIF Orientation tag (values 1 to 8; 6 means "turn 90° clockwise to display") that viewers are expected to apply.
Why it matters
Software that ignores the Orientation tag shows photos sideways, and stripping metadata without applying the rotation first makes the error permanent. Rotating by multiples of 90° is lossless in principle, but many tools decode and recompress on save; truly lossless JPEG rotation needs dimensions that are multiples of the 8- or 16-pixel block size. Flipping a photo containing text makes the text read backwards.
Example
A 4000 × 3000 photo rotated 90° becomes 3000 × 4000. Cropping the original to a centred square for a profile picture keeps 3000 × 3000 pixels, starting 500 pixels from the left edge. Splitting an image into tiles and arranging several into a collage are the same crop and placement operations in reverse.