There is no real reason to populate the city. Even if the city was designed to be transported it was not designed to work without maintenance and unless the mages have truly epic magic it will be essentially inhabitable. It will be a ruin.
That is far from being useless, though. In fact, the mages will become very powerful and rich and quite possibly cause the local economy to collapse.
A fantasy world is a pre-industrial world. The difference is huge. Because, duh, pre-industrial world does not make things industrially on an industrial scale.
Metals are the low hanging fruit. A modern city has positively ridiculous amounts of stuff like copper or steel by pre-industrial standards. Not to mention things like aluminium or stainless steel that would be entirely new.
There will also be lots of glass in the form of windows. All of them will be much higher quality than what a typical fantasy world can produce without expensive magic. And some of them will be larger than any glass pane the mages will ever have seen.
Stuff like rubber and plastics is also of great interest. Whether they can figure out a way to use them is a good question.
High quality building materials. Modern logistics allows us to use building materials exported from the other side of the world. The city will almost certainly have some very pretty and very valuable stone and wood material. We also have synthetic materials to supplement our superior access to natural ones. In a fantasy world many of our floors, kitchen tops, table tops or furniture would be priceless wonders.
What is the fair market value of a stainless steel design chair on a fantasy world? There won't be one is the answer.
The mages will almost certainly just strip mine the city for this stuff and probably lots of other modern wonders, it has much higher value than some empty buildings. This is incidentally that has happened to many ancient monuments. High quality stone is much more valuable than an unused building.
While the mages cannot use or duplicate much of what they find and probably won't even bother as they'll be too busy rolling in gold, they tend to be a curious lot and there will be lots they will want to study.
Books are an obvious attraction to any mage. They'll find lots of books with all those impossibly uniform letters and pictures that clearly were not written or drawn by any tool they know of. Contents of the books aside, they will figure out these books were mass produced in large quantities by machines not men or magic.
What they will wonder about is the why. And if they figure that out the world will truly change. And the riches they have found might be enough temptation some of them will go down that rabbit hole and start thinking about mass produced books and educating the masses. The social structures of a typical fantasy world will not survive that.
And the contents themselves unless the mages strictly censor them to maintain their position. Lets start assuming that the mages, being mages, have magically boosted ability to read stuff written in previously unknown languages. Doesn't really matter that much since they are exactly the type of people who will learn the language even if they cannot use magic. Because mages.
So they'll be able to read this stuff. Describing societies and customs truly alien to them. Describing weird almost magical information technology. Describing advanced mathematics and methods of analysis that might, just might, allow them to truly study the magic at a theoretical level. Methods that allow them to understand how the economies work and make lots of money. Principles of how human bodies work that will revolutionize medicine. Principles of physics and chemistry that will revolutionize their understanding how the world works and make things, including spells, they could not before. And books explaining the steps of how the original builders discovered and developed all that that conveniently doubles as a road map of how to make the good stuff themselves eventually. Like guns or steel mills.
It is entirely possible, even likely, that the mages will strictly limit access to all that potentially revolutionary information. But they will still have it, information is power, and they just got more of it than even a mage really wants.
The mages also have spells and magical abilities. These should allow them to detect that the city came from another world and potentially get some idea of how the transfer worked. They might even be able to use the city as an anchor to create connections to the world it came from. For discreet study and maybe, eventually, actual contact and trade.
They probably also have magic for seeing past events. This will be of great interest in a city built on another world by inhabitants of another world using technologies and methods unknown in your own world.