Metal detectors find metal, how they do this is they send signals into the ground from the coil and when the scanning field gets interrupted by metal the signals return to the coil and then electronics in the detector will tell you about that metal using audio tones, numbers on the screen or meters.
There are different sections on many screens from iron on up, detectors can give you an idea of the target type using something called a conductivity reading...each different metal type will emit signals in different areas when energized because each one registers at different conductivity levels.
Different coins at a few, gold in different areas, silver up high usually but it all depends on the size, shape and more.
It is true you can use discrimination and set your unit to only tell you about certain conductivity readings, or not tell you about them so you can avoid them, but what about other things that come in at those same exact readings which much of the trash we come across does.
Tabs and some coins can be exactly the same, many foreign countries use steel in their coins and those usually come in low just like iron does, big pieces of cans can mimic many coins exactly, gold can disguise itself as a whole slew of other trash metals up and down the range.
One kind of trash metal or iron next to a good metal can combine both conductivity readings and show up on our screens and in the audio as something else in neither area...this is called masking.
Discriminate out some trash and you might also miss good targets too, assume that every perfect coin reading is going to be a coin, or silver or gold, and pretty quickly you will see how wrong you can be.
The universe does not make things as simple and easy as you might hope it would...the laws of physics rule, always.
What you are asking about is if there is a miracle machine, one that can tell you it has found and is scanning ONLY coins, silver and gold but there is no such thing, these are just machines and everything they do is governed by their programming and nobody has been able to come out with one that is that precise.
Invent one and you will be rich in a very short time.
That is the fun and object of this hobby...get all the signals, readings, data and clues that we can and try to figure it all out and make educated decisions trying to dig more of the good ones while avoiding the bad.
Or, digging every signal good and bad which is the highest percentage way to find the most but at the risk of wasting a lot of time or at least using up a lot of your energy.
Honestly if someone did ever invent a detector that actually could only tell you about the good targets and ignore 100% of the bad ones this hobby would not be very challenging anymore and much of the joy and sense of accomplishment would not be there anymore either and those are a few of the main reasons we do this.
We post pictures of our finds on these forums exactly because it is not easy to find the great stuff, it gives us a chance to brag and feel proud for awhile and be appreciated for being successful in a hobby where it is not easy to do and by others that understand all this and the challenges that need to be overcome to have that success.