Rakia Ben Sassi Thanks! to answer your questions:

1. Are you talking about the friend link specifically? You can copy and paste the friend link URL or click the friend link button that you see immediately after publishing an article.
2. No difference
3. It depends on what you write about, but since MEdium has such a large audience, it is possible a paying Medium member will stumble across an article of yours in a search engine. However, for many writers the advantage is increased exposure, reach, and impact on readers, which can in turn lead to other opportunities outside of the partner program.