Readymades were a Duchamp thing completely. Warhol was influenced by them (and Duchamp in general), but he didn't call his work by that term, regardless of similarities in method or concept.
It's been a long time since, I read it, but in Bob Colacello's biography[1][2], I thought he mentioned that Warhol did create (or at least _refer to_) items as "ready made art", without (if I recall correctly) reference to Duchamp.
Warhol would do things like mass-produce objects that looked very much like other things, example: screen printed Brillo Boxes - and then photograph them in say, a super market. It's similar, but different - Duchamp, as the story goes, would just buy the object and call it art. There's some controversy over that, as some of these Found Objects aren't so commonplace, but have key differences in them, to make them more aesthetically pleasing - they may be actually one-ofs, that only look like something commonplace and mass-produced, giving Duchamp the last laugh. That's what happens, when a chess player starts makin' art!
Also see the Warhol's Campbell Soup cans - he just really liked Campbell tomato soup. Speaking of canned things, see, Piero Manzoni's "Canned Shit", where, well, it's what it says on the tin, I guess. I guess, as I haven't actually opened one, myself,
Warhol would use many different mass-production (cheap!) techniques to produce his art. One of my favorites is xerox'd photographs of flowers, he would almost give away for free - certain that they would be both collectable, and that people would simply throw them away, because hey: it's just some cheap photocopy. The one's that survive are worth quite a bit. Incidentally, he didn't get the copyright to the original photograph, so lost money on the whole thing.
I think warhol's biggest, "Readymades" were the actors of his films, who would get little, if any direction and they would just film them doing their thing. See, "Chelsea Girls"
Its worth noting that when poor people (eg, homeless) pile junk outside...its called vandalism.[#,##] When rich, white people do it...its put in a museum. The confusion of vandalism/and art making...is the origin of s0-called "street art".
[##] The origins of the vandalism / art dichotomy more correctly stem from the work of (poor, black) people...ie, graffiti artists...not to suggest they were homeless...my apologies for the "confusion in the allusion", above.
Excellent post, thanks for all those references. Regarding your last sentence, you're right in terms of the actual product, but in art (or at least modern art) it's often the intention behind the product that is important part. In that sense there is a huge difference between what a homeless person does and what a "white person" does, (as you describe it.)
The homeless person is making a mess, the white person is making a statement.
^^^ I'll just put an aside here: I know you were being hyperbolic, but these are obviously quite bad choices; realistically, the homeless person just needs a place to put things. You really want "vandals" and "artists" rather than "homeless people" and "white people."
That said, it simply begins the whole discussion on whether art should be objectively pleasant/obvious/etc... "what is art", we could go on and on.
But I think it's quite well understood these days that there is a lot more to a piece of art than what is in front of your eyes. The message, background, and intention are everything.
As I said though, calling them vandals and artists is the interesting part, since as you point out, the whole street art movement deliberately blurs the line.
I want to say "see also: Banksy", but I don't know if that idea is completely thought out.