The bottom line is, take a coil, for example, as in a metal detector, apply a direct current to it. A magnetic field will form around the coil.
So, while there is no steel object near the coil, it consumes minimal current. And as soon as a steel object enters the magnetic field, the coil immediately begins to "eat" more current, since it attracts a piece of iron to itself.
So, if we had a very sensitive ammeter, we could look for metal, including it in the gap with the coil. Only there measurements should be in milliamps, probably ... When the object is clearly in the center of the coil, the device will show the maximum current consumption.
And now about non-ferrous metals. When any metal enters an electromagnetic field, induction currents arise in it and, as a consequence, a magnetic field. For example, if you hang a coin on a thread and swing it near the magnet, it will always turn around on a certain side to minimize the "resistance currents", Foucault currents. Similarly, when we drive a coil over a non-ferrous metal, currents and an electromagnetic field will appear in the metal, which will “repel” the coil, as a result, it will also have to consume more current.
So the main question is, how can one make such a very sensitive ammeter, maybe based on a multimeter?