what happens when you follow another user?

Nothing? I think you can do the same thing like mail and stuff even if you don't follow the m

you get notifications of their timeline performance like when they update their reading lists and stuff. but it doesnt show up on the notification button like when someone replies to your comment or when your manga updates.

If I follow someone, where should I go to see notifications of reading list updates and stuff?

Press on timeline under your menu