Do I get it right that a font has to be installed on the users device to be shown?
Or is it possible to provide fonts via XWiki (as described in “Upload locally custom FFT file”) so that the browser shows the correct one.
So how this works in brief words. If the user has the font: they will see your design as intended. If the user doesn’t have the font: their browser defaults to a similar (normally such font is set by default i.e MS Edge edge://settings/appearance/fonts). If user’ browser doesn’t have such font than a system OS will be used (but this rare case)
There’s a previous discussion Using brand font: only one box is behaving differently - Help / Discuss - XWiki Forum where some clarifications were provided.
By the way, if you think that FAQ page lacks details, feel free to contribute by improving it.