The first step is to design what you want your garment to look like. Your design should include a template of a basic human figure. The clothes can be drawn to fit on top, then the lines can be erased, but if you are good at drawing then the clothes can be included in the actual drawing. Most good artists don't do holiday sweaters. That task is designated for the retired or just tired designers.
Many designers make side notes on their sketch as to what fabric, texture, and additional details they want on their garment. These are called specs. Specs are specific details written on the side of the drawing with an arrow pointing to where the detail is to be included in the design.
After designing whatever is to be made, the next step is to discover what fabric is going to work best for that specific garment. Some fabric won't work well for short people or poets, etc. For ugly sweaters, the typical selection process includes a 'skin test', which puts a variety of different wool on monkeys to not only gauge their comfort, but at what speed they move when in the sweater. Speed and comfort are critical when designing ugly sweaters.
The next step is to create the pattern, which is a whole other skill in itself. Then, the pattern is applied to the fabric to be MARKED (marking). This is the process of the pattern being transported to the fabric. This is usually done by placing a carbon sheet, FACE DOWN, on the fabric and then placing the pattern on top so when you mark, the lines will show on the fabric. Then, the fabric is cut. Each of these steps are completely different professions.
From there, the fabric is constructed or sewn together. This is usually done by an outsourcing agent, where the labor is cheaper. Then sent to the manufacturer to tag it, package it, etc. This is the time when the product is being marketed. The product is then shipped to stores for people to buy, where the price includes labor, overhead costs, shipping, import/export tax and usually a 40% markup on the total price of those things so that the manufacturer can make a profit.
We have no idea where our sweaters come from. We can only assume that they were created with the utmost ethics and support for poor people.