Medium assigns a unique ID for each paragraph. These can then be individually targeted as anchor links inside the URL. For example, a URL like the one below links to the first subheading of a Medium essay:
https://jjosephmiller.medium.com/the-rhetoric-of-content-types-c9e6994d2b01#4299
That said, Medium has not made it particularly easy to target specific paragraphs. As of the time of this writing, the only way to construct such a link is to view the source code for the page, find the ID in the HTML tag, then add it to your URL.
Annotating online is largely a kludge. But there are a few places trying to do it better. Medium’s inline comments are brilliant in this regard: Highlight a passage and add some marginalia. The comment sticks directly to the highlighted passage and automatically generates a running list of all those comments in your own account. It’s marginalia and a note taking app all in one.
And thanks to the Internet’s magic robots (more formally known as Application Programming Interfaces, or APIs), you can push different content combinations to different platforms. You might send a photo and some tags to Instagram, combine those with a factoid and send them to Twitter, or send the photo and a short summary or pull quote to Facebook. Package a longer set of paragraphs with some images and charts and send those to your blog and to Medium.