Can't doers not only don't understand manufacturing, they don't understand economics either. A basic principle of economics is replacement. Everything has an alternate use and everything has a alternate replacement. For example, corn is used for feed but also for making ethanol. Other things can be used for feed or ethanol. The market determines which (unless politicians get involved.)
What is modern technology? It's specializing and distributing manufacturing for greater efficiency. It is dependent on quality tools to produce the next level of product. But everything produced is done by a person with the skills. What products can be made is more a result of design than capability because good designs make more things capable of being produced.
Forging is a production method that goes back to the earliest times yet is an element of modern production. Forging involved adding pressure to a material (anything from steel to plastic) that is often heated to below liquid temperatures. Pressure can mean hammering, rolling or pressed hydraulically or by screw. Extrusion is a type of forging for creating tubes, wires and beams of any shape or length. It doesn't require modern technology and the machines themselves can be designed to be easily built and repaired.
Unless we're making wire we can ignore the puller and winder. For metal beams (of any shape) cooling just means having a long enough bed for the beam to air cool on. For metal (which holds it's heat) we don't really need heater bands but they are simple enough to include (and you would not want metal cooling in your extruder... what a mess.)
The heater coil is simply a metal turning electrical resistance into heat. For metal extrusion, you don't use a screw as pictured. You use a hydraulic ram. Rather than a hopper (which allows continuous operation... a modern technology!) You would start with a liquid metal cast into a cylinder of the right size for input into the extruder. An extruder can be made up of simple parts and produce beams one at a time, but at a fast enough rate to support a martian industrial ecology. Yes extruders can be really simple.
Having a little bit of capability over time can lead to modern technology levels of production and quality. It's not magic. Extruders themselves are not required to make beams. They just make it a lot easier.
Any shape can be produced depending on the die and closed shapes can be made of open shapes with a couple of simple additional steps.
How do you make a hydraulic press? Do I really have to explain a lathe to you? It is used for making both the tube and the cylinder. You first extrude the cylinder and machine it on the lathe to a required tolerance. You don't actually have to extrude the cylinder because a lathe will also work with just a hunk of metal, but that's very inefficient and wasteful. You also machine the tube on the lathe which can be multiple parts welded together after machining. This also doesn't require any magic modern technology. These parts, all in a days work, provide some of what you need to make a tractor. If you need a dozen tractors, they could all be produced in less than a year with the few dozen colonists available (not all will be involved.) Final assembly from all required parts takes less than a day for each tractor in your shirtsleeve mars garage. Many garages are made possible by these tractors.
BTW, 3D printers are a type of extruder that can replicate itself (with a few extra machined parts.) They do require a computer to operate, but the electronics of the printer itself can be martian ISRU made discrete components.
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