Does Sql Server maintain the free space defined by fillfactor?

I'm new to concept of what fillfactor is and why it is used. So far, reading [this article](https://www.brentozar.com/archive/2013/04/five-things-about-fillfactor/) , I've learned that by setting the fillfactor configuration of an index, we specify how much space on each page should remain free, so that whenever a new record needs to be added to a page, there will be some free space for it. My question is that: 1. Is that free space on each page, intended to remain free? I can guess that, that free space is used when an insert happens, but later the database engine, moves data, so that again we have some free space in each page. Is this what happens in a DBMS?