A WOFF2 whose outlines are PostScript (CFF) unwraps to an OTF you can install on your computer or use in a design tool. A WOFF2 made from TrueType outlines should be converted to TTF instead.
The outlines and tables of the font are copied unchanged and only the wrapper around them changes, so the letterforms, hinting and layout features are exactly the same. WOFF2 is handled by a WebAssembly module that downloads the first time you open WOFF2 and is cached; the font is expanded back to plain tables. A web font holds either TrueType or PostScript (CFF) outlines, and OTF can only be the matching kind. If the choice is wrong, the conversion stops and says whether TTF is the right one. A file that is not a TrueType, OpenType, WOFF, WOFF2 or collection font is refused.