I'm new to Blender and currently watching tutorials on modeling. In this video, at 6:50, the person adds a Plane mesh that is already in a octagon form; it has 4 vertices but looks like an octagon. I've been trying to find out how he did this but can't find out the answer.
The guy does not have Screencast Keys enabled, but if he did, you would know that he keyed Ctrl + 1 (1, not Numpad 1), which is the keyboard shortcut for adding a Subdivision surface modifier immediately after adding the plane.
If Ctrl + 1 doesn't work for you, you can go to the Properties panel > Object modifiers tab and add a Subdivision Surface modifier manually. Even if you use the shortcut, you still need to tweak the settings in that tab. A normal Subdivision Surface modifier looks like this: