Changing the shape of a plane

I’m new to blender and currently watching a tutorial on how to model, and in the video it shows the person putting in a plane, and it has 8 faces to look like an octagon (It only has 4 vertices at the corners). I was wondering how do I put in a plane that already has 8 faces? Adding in a circle and changing the vertices to 8 doesn’t make it have 8 faces but only 8 vertices.

I’m new to blender and currently watching a tutorial on how to model
Are you going to tell us what tutorial ?
Are you going to show us what you mean ?

and it has 8 faces to look like an octagon (It only has 4 vertices at the corners).
What does this mean

Please don’t make us use guesswork !!! If you have problems and what a solution it’s down to you to do most of the work, make it as easy as possible for other people to help you

Sorry, I read in the guidelines that new members can’t post links, and since I just made an account a couple minutes ago, I wanted to stay within guidelines. https://www.youtube.com/watch?v=KPWNn2PhXYs is the video, and what I’m talking about is at 6:50 ~ 6:58