[OlegWock]

I think that might depend on particular font, but from what I tested, manually downloaded fonts do include OpenType features like smcp or swsh

[iggldiggl]

Sometimes they might also ship outdated versions, e.g. the last released version of Fira Sans is 4.301, but Google still only serves version 4.203. Plus technically Fira Sans has been superseded by Fira Go, which includes additional scripts, but Google doesn't offer that one at all.