The trees pictured are almost certainly stem cuttings. As mentioned, Seiju root quite easily. Layering is too labour intensive for commercial growing except for special types and larger material.
Seiju have a tendency to long, straight growth. Pruning is very important to introduce both taper and interesting trunk shapes.
Cuts will thicken if you allow multiple shoots to grow from the site. Need to be vigilant and remove excess shoots early, before they start to expand the area. I noted several spots in the group where there are 2 or more branches growing from the trunk - watch those spots and be prepared to remove one or better, get rid of excess now, before the problems start.