How do I use them in Google Docs, Slides, or Sheets?

Your best option for inserting icons into any Google Drive file is to use a PNG as the other file types included in the set are not supported at this time. If you intend to print the documents you create, you should use one of the larger 256px or 512px PNG sizes and scale it down to the size you need. This will help make the icons appear more crisp and sharp when you print them out.