EXIF orientation tag values:
1 = Horizontal (normal) ← Camera grip at right as seen by the photographer
2 = Mirror horizontal
3 = Rotate 180
4 = Mirror vertical
5 = Mirror horizontal and rotate 270 CW
6 = Rotate 90 CW ← Camera grip down
7 = Mirror horizontal and rotate 90 CW
8 = Rotate 270 CW ← Camera grip up
A value of 1 basically means that the reviewing/opening app should do nothing. When an image is rotated and exported, the output file’s orientation tag should have a value of 1 in order to not be rotated by the viewing/opening app.
Also, the camera can be set to automatically rotate the built-in preview and thumbnail or not. This, in conjunction with the orientation tag value and the orientation set by an app, creates whatever orientation value(s) come up in a sidecar, values that don’t necessarily match the EXIF tag values.
Depending on how apps handle these tags, orientation, as seen on screen, can be almost anything. Example: RawDigger ignores orientation info completely.