Following a user means you’ll see more of their activity (such as posts, updates, or discussions they start) in your feed across the community. It’s a personal connection that doesn’t depend on which spaces you belong to.
Joining a space, on the other hand, gives you access to all the content, discussions, and resources within that space. You don’t have to follow every member of a space to see its updates — being a member of the space itself is enough.