I am a completely new user and a little confused. Do changes to a jpg file made in the Develop permanently change the original image? I presume they do. So I will have to keep the original file separate from changes.
No, they don't get changed. Lightroom stores the edits in its catalog, not in the images. That applies to any image format that Lightroom supports. It also means that if you want to use the edited jpeg outside of Lightroom, you need to export a new jpeg.