I will try to explain a few things:
are bolted to the cylinder in between the rasp bars. They were originally meant to harvest corn, to keep the cobs from going inside the cylinder, where they do not do anything. The filler plates keep the corn cobs in between the rasp bars and the concave to get the kernels rolled off the cob.
In wheat it is generally suggested to remove the filler plates. Since the wheat head is attached to the straw it can not get inside the cylinder. But the gap between the rasp bars allows the crop matt to fluff up before it gets hit again by the next bar. That is similar to old age hand threshing with the stick.
Some operators run stripper heads. In this case only the wheat head goes into the combine and the straw is left in the field. Filler plates are required in this case to keep the wheat heads between cylinder and concave.
are placed in the rear of the concave, where the wires end. By filling the big holes in the concave, the crop mat is kept in longer and therefore threshed longer. I have never tried this in wheat, but it is necessary in high moisture corn. High moisture corn needs to stay in the concave longer, because the kernels are tougher to get of the cob, what takes more threshing time.
In dry corn it can vary. I have been in corn varieties, where I had to fill the hole back side of the concave with inserts to keep the husks moving instead of plugging the rear of the concave. In other varieties I had to take all the inserts out to keep the kernels from going up onto the walkers.