As you would know, LaTeX uses ` for opening quotes and ' for closing quotes. Most fonts also allow the usage of " for closing doublequotes. My question is: what are the differences in typesetting, formatting, etx, between two 's and one "? Is it a meaningful difference like using dollar-signs against using backslash-notations? Or is it just a personal preference thing for the same function?
" is not supported LaTeX syntax, it happens to work in some font encodings. As it is not (supposed) to be used as character input it is often used as a shortcut character in many babel package and other language settings eg
"- as a variant hyphenation construct not suppressing hyphenation of the following word.
" does produce a double quote Then it (should) be a straight left-or-right quote, it is just an accident of the legacy TeX encodings that the right double quote was in that position. If you want single character input it is better to use the standard Unicode left and right double quote characters U+201C U+201D