When you unfollow someone on Facebook, you stop seeing their posts until you follow them again. The posts of that person will never show in your timeline. See why people unfollow others.
Unlike blocking, unfollowing a person keeps them on your friend list. Nothing will seem to have changed for the person you unfollowed and they won't know that it happened.
When you unfollow someone on Facebook, they don't get a notification. When you follow them back they don't get notified as well. The whole process is transparent to the user.
The person that you unfollowed will still see your posts normally because they are still following you. Unfollowing someone doesn't prevent them from seeing your posts. See what happens when you block someone on Tinder.
If you post often on Facebook, but never interact with the posts of someone because you unfollowed them, they might guess that you unfollowed them.
When you unfollow a person, you will still get their birthday notification normally. Unfollowing only affects the posts on the timeline.
The person you unfollowed can still interact with you normally and send you messages. All of Facebook's functions including commenting, tagging, sending messages and invitations keep working normally.
Everything remains the same for the person you unfollowed, as there would be no change in the way Facebook works for them after unfollowing them.