Today, Levi's 501 jeans are made globally, but in the 1990s and a few years into the 2000s, the Levi's 501s were made in the USA, at their San Francisco factory.
The main difference between the current 501 and the vintage 501 is the thicker denim fabric. Overall, the jeans are heavier, and some people prefer this. Over time, with use, the pants fade and become extremely soft. This old material is similar to the current Cone mills denim, but was a little thicker and heavier.
Classic 501s also shrink-to-fit, so the practice was to wash them in cold water, first, to shrink them, and then wear them around until they shaped to your body. Over washings, the pants would develop a customized fit. Over time, they would become soft. A little bleach could also be used to fade and soften them further, but it would weaken the fibers and reduce their longevity.
Though the fabric is thick, it's comfortable. The weave seems to allow the pants to breathe. This isn't unique to Levi's, but not all brands have this quality. I've had thick pants that were uncomfortable and sweaty.
A pair of pants could last a half year to a year, or more, even if they were worn weekly. Though they cost more than other mass market jeans, they were a good value.
If you happened to like wearing old jeans (as I did, and do), you'd patch them up. Eventually, you'd need to turn them into cutoffs, and use the leg scraps to patch the seat. By this time, the pants would be double their original thickness, and soft as a cloth diaper.
I have a few different vintage pants for sale. None are in new or fine condition. They all have signs of wear.