I found this to be the easiest part of the assembly process.   The extruder is the mechanical device that pushes the filament all the way down to the hot end.   The extruder proper is a simple stepper motor, attached to the top plate of the 3D printer and pushes the filament down a Bowden tube.

Here’s a picture of the extruder before I attached it to the top plate.

From the picture, you can see the motor and the extruder proper attached to the motor.  The filament is inserted at the top and is pushed at the bottom.

Here’s a picture of the extruder and the Bowden tube connected to the hot end. For display purposes, I have reversed the arms of the hot end.

That’s pretty much it for the extruder, quite easy.