The farmers I know, have a huge range of skills, from metalworking, construction, woodwork, mechanical engineering, biology, geology and are putting that knowledge into practice every day. All of that while planning years ahead (for instance how to plow your field this year, so that in 5-10 years you don't get a massive slope on your field's edge).
So when farmers talk about fixing a tractor themselves, it's not just about being able to buy some screws or replacement joints, it's about welding that piece you need, sanding its steelframe to remove rust, knowing whether to bend or clinch instead of welding. It's even about building the tools to build those parts.