Regarding your question on how it works....
The button you push is attached to a small rod. If you look at the side of the bolt carrier there are notches cut into it. When you push the button the rod is pushed in and catches the notches and pushes the bolt carrier forward. It's angled so it will push forward instead of sideways. It is there to assist in getting the bolt all the way the forward.
The only time I've ever had to use it was one time I dropped my 20" into very loose sand in the woods and the dust cover was open. Sand got inside the upper and wouldn't let the bolt go forward. Tapped the FA a couple times and it went on forward. Also, the rifle operated just fine even with the sand in it and the magazine
ETA: If I read right, you don't have an AR yet so here are pics of what I'm talking about.
This is the forward assist not engaged:
Forward assist engaged:
Finally, notches in the bolt carrier that the small rod seen above pushes on:
Hope this helps and welcome to the sickness.