A photo that appeared upright on your phone and sideways everywhere else is a common and slightly maddening problem with a simple explanation.
Cameras often save the image in one orientation and add a note saying which way up it should be displayed. Software that reads the note shows it correctly. Software that ignores it shows the raw image, which is sideways.
That is why the same file can look fine in your gallery and wrong once uploaded to a form or opened on a different computer. Nothing changed about the photo, only whether the note was honoured.
Rotating and saving fixes it permanently, because the pixels themselves are rewritten the right way up. After that every piece of software agrees, with no note to ignore.
The site is ignoring the orientation note your camera saved. Rotating and saving the file removes the ambiguity.
Turning by ninety degrees does not. Small angle corrections require cropping the corners.
No.