I took a closer look at the pen.
I tried the modification of a BufferGeometry some time ago.
However, only the modification of a vertex, an edge or a triangle.
Modify indexed BufferGeometry (mouse or input)
see modify Geo
For my current project, I’m going to try to combine the two things, i.e. also include the nearby vertices in 3D. Possibly by adding the x, y, z keys to the mouse movement to define the orientation.
That will certainly take some effort. 