Let's put the selector switch together. Although it's more complex, the basic principle is that of a flat triangle attached to an axle. The triangular portion is made of several layers. The axle is split up into three sections, with the middle one being U-shaped.

Why is it U-shaped? It protrudes at the bottom in order to interface with the trigger. It has a gap in the top to avoid blocking the movement of things such as the bolt carrier group (when inserting and removing the BCG, of course. The BCG doesn't come back this far during cycling).

I would recommend building it in three parts as pictured, because this will make it easier to insert into the receiver. Put the left side of the axle in first, from the inside. Then attach the right side with the triangular switch from the outside. Glue them together and attach the axle left cap on the other side.

As the text says, you can't just build the complete selector and try to put the whole thing into the receiver. On a real AK, the selector hole has an elongated opening, through which the U-shaped portion of the axle fits. Here, the U-shaped portion is larger anyway, so the advantages of enlarging the hole aren't worth compromising the structure.