You do not say which Language or operating system or version of LibO you are
using, however in general terms I hope the explanation below helps.
LibreOffice only uses Unicode, that is to say UTF-8 in your terms. The
characters you are seeing are actually Unicode. This provides support for
about 108,000 characters. US-ASCII uses the first 127 of these. So, if you
go to INSERT > SPECIAL CHARACTERS the first (decimal) 127 or (hexadecimal)
U+7F characters are the same. The double left and right quotation marks are
down in about U+201C. The double quotation is (decimal) 34 U+22 is also
available in US-ASCII.
If your particular problem is DOUBLE QUOTATION Marks, the variation is
Country / Language dependant. But you will find assistance using the HELP >
QUOTATION MARKS. You need to turn off the AUTOCORRECT OPTION if it does not
do what you want. I quote from the HELP PANEL.
To Stop Replacing Quotation Marks
1. Choose Tools - AutoCorrect - AutoCorrect Options.
2. Click the Localised Options tab
3. Unmark the "Replace" check box(es).