If you're in the Substack feed, you may notice "subscribe" on a note or "subscribe" when clicking on someone's Substack profile.
If a writer has a publication and you click "subscribe", you'll be automatically subscribed to their publication and also see their notes in the Notes feed.
If a reader doesn't have a publication, you can follow them on Substack. Following a profile means that you'll see their notes and reading activity such as likes in the Home feed.