“Minhag” happens when a cities Halachic authorities approve a specific practice for a Halachic reason.”
It seems to be that you are confusing two different concepts. Minhag has a couple of components, one is Halachik Minhag, which is if a certain place has accepted a certain psak etc. And then there is the concept of a Minhag , מחמת “גדר, they are totally different and have different conditions as well.
E.g a Minhag based on Halacha can be easily overridden in many cases where it was proven that whatever it was based upon was in error, it can be abolished. Whereas, if it’s based upon מנהג למגדר מילתא is much harder to override. In some cases you will need a בי”ד הגדול במנין ובחכמה ,etc. It also depends how it was implemented and how the Kabalah of the minhag was accepted – was it accepted by all? Was it for the time being or leDoros? Did all accept it for their kids and kin, etc.
Just because people do or wear something does not define it as a Minhag…
In any case, the subject of how and when a Minhag is applied is very complex, each and every case and place is definitely different.
For all who want to get somewhat familiar should learn through the פרי חדש סי תצ”ו in Hilchos Yom Tov. He wrote an entire kunteres going through all different scenarios.
There is a sefer written by the great Gaon Rav Dovid of Karlin called עמק ברכה, which he wrote regarding Jerusalem’s Cherem on learning secular subjects, he claims that Jerusalem has no basis of a Minhag in that case. (It should be available on Hebrewbooks..)
And in our case, there is no Minhag העיר in most cities.