A video looks fine on your phone and lies on its side everywhere else. The fix is to turn the picture itself, so every player agrees.
Phones usually record the picture in landscape and add a small note saying which way up it belongs. Players that read the note show it upright, and players that ignore it show it sideways.
Turning the picture itself removes the disagreement. The cost is that the video has to be encoded again, which takes a while in the browser, so keep the original in case you want it.
For a mirrored selfie video where text reads backwards, use Mirror left to right instead of a turn.
Yes.
If the top of the picture points left, turn it right. If it points right, turn it left.
No. The new copy is made on your device.