a real reshut harabim d’oraisa, on the shitos that require 600000, seems impossible to enclose with an eruv. but chazal also talk about genuine enclosures, walls with gates that lock at night, like yerushalayim had. so:
1. if a huge city like manhattan actually had walls with gates locked at night, would that fix it even with millions of people inside, or once a place has 600000 does no door in the world help?
2. for an island town with water on almost every side and only a couple of ways in, does the water do any halachic work without real gates, or is it just a wire on poles?
3. and a landlocked city with no natural boundaries at all, a st louis or a kansas city: does the eruv there have to enclose the whole city, or can a neighborhood inside a reshut harabim become its own reshut hayachid?
4. bottom line: should a person carry relying on the shitos that allow these eruvin, or not carry because if it is a real reshut harabim every step is an issur d’oraisa?
Thank you Rabbi, thank you very much.
Yes. The basic distinction is between a real mechitzah and an ordinary eruv made with tzurat hapetach, the familiar poles and wire.
If a genuine reshut harabim were actually surrounded by proper walls and gates in the halachic sense, that can change its status. The fact that 600,000 or even several million people live inside does not make real walls meaningless. Yerushalayim itself is the classic model discussed by Chazal.
What does not work, according to the stricter understanding, is taking a definite reshut harabim deOraita and turning it into a reshut hayachid merely by running an eruv wire around it.
Water can sometimes provide part of the enclosure, but only where the shoreline itself creates a halachic mechitzah, such as a sufficiently steep bank or drop. A smooth beach gradually running into the water is not a wall merely because there happens to be water there. So an island is not automatically enclosed for Shabbat.
A city also does not necessarily have to enclose every square mile. A neighborhood can have its own valid boundaries and become an enclosed area even though the larger city continues outside it. The question is whether that particular area has the halachic enclosure it needs.
So an eruv certainly works in ordinary cities and towns when it is constructed properly. The concern is specifically where the streets are truly reshut harabim deOraita according to the opinion you are following. In that case, I would not rely on a simple wire and poles to erase a Torah-level reshut harabim.
That is why the status of the streets underneath the eruv matters just as much as the eruv itself.