Here's a brief historical etymology. Long before the Turks arrived, there were numerous variants of the phrase eis tḕn Pólin, “in the city”, and a shorter slang variant "stanbol" used by the locals. It helps ti keep in mind this was a center of trade for over 1000 years, so there were a lot of people of different races who converged on the city in all this time. Most were a combination of Byzantine Greeks, Slovenians, and nomad traders who lived there long before the fall of Constantinople in 1453 by Mehmed II. Before that time, the Ottoman's set up their capital in Anatolia.

Mehmed II was well educated in trade and rather than sack the city as the Roman Church did in 1204, he spared the citizans so the trade could resume after the seige as soon as possible. As soon as word spread that the Turks were willing to do business, a migration of money changers hit the city like a tidal wave and the city thrived for the next few hundred years, with the Ottoman Turks getting a middle-man tax percentage for guarding the silk roads. The former Byzantine merchants, along with Greek, Slavic, and Jewish, referred to the city as Stanbul, and Ottoman historical documents support this. (Meanwhile, as the great Catholic Inquisition was terrifying western Europe and still referring to Stambol and Constantinople for several hundred years, the city took on 4 names from the late 1400s through the 1800s: "Stambol* by non-Muslims, Is-stanbol (meaning Islamic) by Muslims, and Constantinople by Europeaners, and even Konstantinòpol by a mix of others. In the west, we're taught it is a corruption of the original Greek, and perhaps it is, but there's no difinitave documented proof because even the Lycinian phrase for "Golden Horn" and the vikings had a name very different. So the jury will always be out. All of these names existed because of trade and the city was named and spelled traders doing the shipping. (Kind of like today with flying, "LAX-IST" means your bags will depart from Los Angeles Int'l airport and the destination is Istanbul-Attaturk Int'l Airport.) All these variants hung around for centuries until after WWI, when the Treaty of Lausanne - as proclaimed by the new Turkish Republic, recognized the city internationally as Istanbul once and for all. So that's the whole story squeezed down into one bite.