JPG vs JPEG: is there any difference?
None. JPG and JPEG are two spellings of the same file extension for the same format. Why there are two, and what JFIF files are.
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In short: There is no difference. .jpg and .jpeg are two extensions for exactly the same image format. The three-letter .jpg exists because early Windows only allowed three-letter extensions. Renaming one to the other changes nothing inside the file.
Why two spellings
JPEG is named after the Joint Photographic Experts Group, which wrote the standard in 1992. MS-DOS and early Windows limited file extensions to three characters, so .jpeg became .jpg. Macs and Unix had no such limit and often kept .jpeg. Both stuck.
What about .jfif?
JFIF is the file layout most JPEG images use, and some versions of Windows save images from a browser as .jfif. It is still a JPEG. Renaming it to .jpg is safe, and so is converting it; either way, a site that insists on .jpg will then accept it.
When renaming does not work
Renaming only works between names for the same format. A WEBP or HEIC renamed to .jpg is still a WEBP or HEIC inside, and a site that checks the contents will reject it. Convert those properly instead.
Questions
Is JPEG better quality than JPG?
No. They are the same format; quality depends on how the file was saved.
Can I rename .jpeg to .jpg?
Yes. Nothing inside the file changes.