Hello,
Embedded fonts are fonts included in the DOCX file, not the default font: https://www.howtogeek.com/106681/how-to-embed-fonts-in-a-microsoft-word-document/ . LibreOffice works with embedded fonts too.
replaceVariableByText keeps the font-family used by the placehoder in the template unless the whole paragraph is replaced (type option of the method). replaceVariableByHTML doesn't keep the font-family of the placeholder, it adds a default font-family, so you need to set font-family to use a custom one.
phpdocx 10 added the stylesReplacementType option to keep existing placeholder pPr and rPr styles such as font sizes, font families and others when transforming HTML.
Regards.