GAON:
“Milhouse is basically correct, there are many poskim who clearly state that the very concept that a צורת הפתח is not sufficient to permit in a רשה”ר is only a d’Rabonon. (See Shulchan Aruch haRav 364, Avnei Nezer hil Eruvin, Aruch Hashulchan. )”
Actually most poskim maintain that a tzuras hapesach is sufficient on a d’Oraysa level. However, as I mentioned some Rishonim and poskim maintain otherwise, including the Bais Ephraim (who admits that according to the Rashba re Rambam, and the Hagaos Ashri a tzuras hapesach is effective me’d’Oraysa) and the Chazon Ish. So its not as simple as Milhouse argues.
“Many here are confusing by saying “Eruv” with eruv Chatzeros or a tzuros haPesach of 4 lechis only where we say אתו רבים ומבטלי מחיצות at least מדרבנן.”
I agree.
“As per Chazon Ish, most cities nowadays are by default a reshus hayachid, as most streets that are basically 2 mechitzos עומ”ר end up at one point in the city within a third wall, thus rendering it as a true reshus hayachid, and therefore permitted with a tzuras haPesach. in any case, brooklyn is surrounded by at least 3 mechitzos בנ”א by sea walls and gates עומ”ר and are not מפולשין ומכוונים due to the layout of the streets.”
I agree on both counts. Most people cant make the distinction that we usually don’t need to rely on the CI”s chiddush in large cities today.