Struggling to Understand Vertices Behaviour 2 This is the second time I am posting this as I was unable to post the screenshots above the first time. I apologise.

Blender Guru Anvil Tutorial

In the above link, the tutorial has a step a minute before the end in which he completes an extrusion with a clipping mirror modifier. At this point, the vertices at the ends of the semi-circle on the bottom of the anvil become locked to their actual vertex locations, instead of being near them due to the shade smooth and subsurf mod. I am unsure why this is occurring. Any help would be greatly appreciated.
you probably did press E and then Esc. That always results in duplicate vertices. If you dont undo that step and just carry on, you will be left with duplicate vertices in your final mesh, which will always break subsurf (will make a hard edge with lots and lots of duplicate vertices)

To fix it select everything and press Alt+M and select "by Distance".