I'm trying to extract the files as a 7zip, but it keeps saying that it "cannot extract these files as an archive". Am I picking the wrong files or something? Or am I doing it completely wrong?

You don't extract files from a zip to upload, you have to put them into a zip, z7 or rar archive and upload the archive to the Nexus.

Check the archive is not corrupted by putting it in it's own folder and unzipping it.