In addition to extrusion, you’ll do some basic finishing operations such as sending it through some rollers to make it as circular as possible. Temperature control at every stage is critical.
You need to prevent any gasses or moisture from being allowed into the plastic as you form it, too, which makes the apparatus still a bit more complex compared to a regular old hot end.
The challenge in making filament is largely QC. The tighter the tolerance, the more the filament you produce is worth, which is why it’s still best to get the filament from a professional manufacturer at this point, and will probably remain so for quite a while.