Today I learned that the Salesforce Lightning UI doesn’t have a it’s own font, or a specific default font, but it is using the system default of the operating system, it is running on. I think it is a smart move - Microsoft, Apple and Google invest enough resources in these fonts, so why reinvent the wheel?
For Windows 11 the font is Segoe UI Variable, for macOS San Francisco.

SAP Salesforce Integration with Vigience Overcast macOS on top, Windows 11 below