Linseed oil comes from flax seeds that have been dried and pressed...and then processed over and over. The process ultimately takes years, as the oil stabilizes into a form that will not yellow after it's used in paint. Oil painters have been using linseed oil-based paints for centuries, but these days we tend to forget how exactly that paint gets made, and how much labor is involved.
In the video below, painter Joe Besch shows his process for making linseed oil, and ultimately paint (which is basically the oil plus some pigment). I found this video extremely relaxing, mainly because I wasn't doing any of the work. I was a little shocked near the end when I realized his first batch started in 2014 (and he doesn't have much of it left). Anyway, enjoy:
Besch explains a bit more about the process on his website.