New York's "Miracle Mile" is a stretch of Northern Boulevard in Nassau County (but fairly close to Northern Queens). Indeed, I think it was in George Santos' mixed Nassau-Queens district. Might be wrong about that, but it's close if not in. It is dotted with luxury stores because the area is fairly rich. It also used to have big department stores (I seem to recall a Filene's Basement and a Lord & Taylor back in the day)
Chicago's Magnificent Mile is a stretch of Michigan Avenue running from about Oak Street Beach on its north end to the bridges over the Chicago River at the south. It includes a number of landmarks, including the John Hancock Tower, the old Water Tower, and Water Tower Place, which is one of the several malls in the area. Michigan Avenue is also dotted with many luxury stores and the whole place is certainly a tourist standard for visits to Chicago.
In Trump's defense, it is fairly common for New Yorkers in Chicago to confuse the two expressions.
What Trump is referring to is the occasional "teen takeovers" on Michigan Avenue - this is when large numbers of teens, usually from the South Side (read: black) suddenly materialize in areas like River North, Millenium Park, or, yes, the Magnificent Mile. They are rowdy, traveling in large numbers (sometimes several hundred) and some of them are certainly intent on shoplifting and other crimes. Many are just there for the fun, it should be added. This is, of course, the nightmare scenario in Trump/MAGA mythology: the luxury "white area" suddenly beset by young black kids! So Trump wants to highlight the teen takeovers and wants Fox to show you images of them. Black Kids Run Riot in front of Ferragamo and Tiffany's, oh no!
The idea that these takeovers (which have largely subsided anyway) are a problem that require troop deployments is, of course, completely insane.