biblatex: postnote behavior

biblatex will normally detect if the postnote argument of a citation is a single page or a page range and will add the appropriate prefix. This also works for, e.g., Roman numerals, but not for numbers plus text, as in your third citation. In these cases, you have to manually add \pno (which prints the single page prefix of the current language) or \ppno (which prints the page range prefix). Using a non-breakable space, you should write your third citation as

``Some \emph{quotation}'' \citep[][\pno~43, emphasis added]{john-73}.

See section 3.14.3 (Page Numbers in Citations) of the biblatex manual for details.

From biblatex 3.13 (2019-08-17) onwards, you can use the macro \pnfmt instead of \pno and \ppno. See https://github.com/plk/biblatex/issues/870.

\pnfmt formats its argument exactly like biblatex formats the complete postnote argument. So if \pnfmt contains only a page range and the postnote format adds a page prefix, you will get a page prefix from \pnfmt. In that case \pnfmt will automatically choose the correct form of the page prefix ("p."/\pno or "pp."/\ppno), add the required space and normalise the range delimiters. Furthermore, the command is useful for uniformity in case you decide to modify the postnote field format.

\documentclass[british]{article}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage{babel}
\usepackage{csquotes}

\usepackage[style=authoryear, backend=biber]{biblatex}

%\DeclareFieldFormat{postnote}{\mknormrange{#1}}

\addbibresource{biblatex-examples.bib}

\begin{document}
\autocite[\pnfmt{380-382}, emphasis added]{sigfridsson}

\autocite[\pnfmt{381}, emphasis added]{sigfridsson}
\end{document}

See also Biblatex citepages=omit not working when postnote has text.