Basic question on the mechanics of an eruv. If a karmelis is surrounded by a tzuras hapesech, that closes off the city. Isn’t there still a problem of going from one persons property to another property owned by someone else unless an eruv is made. Does the Eruv work if the people using the Eruv know nothing about the eruv joining all the properties?
Yes, there is still a problem of going from house to house. Although the surrounding eruv makes the entire area into one chatzer, it remains forbidden to carry from house to chatzer, unless an eruv chateros is made. This is done by sharing a meal (often, but not necessarily, a box of matzos, because it lasts long): a kinyan is made, giving all residents a share in the matzos, and the matzos are kept somewhere in the area. There are various problems concerning non-Jews, or secular Jews, which are dealt with by having some rights to their property (agreement with police, or with municipal bodies).
The eruv helps even if people don’t know about it.