Of course, metal detector can detect metals and there is a technique to detect WHAT kind of metal is. I think it's something called "phase discriminator". Different metals have different characteristics in the phase.
I have only read that technology in a book a lot of years ago. My experience in electronics is very little, so I'm not experienced in electronics. I have the plans, so it will be easy for a person that has experience in electronics. The circuit is simple. It's not very sophisticated.
This device I explained in the last post (the one that is like a radio), seems to work by "detecting frequencies". I mean, in an ordinary radio you can tune a specific frequency by varying the capacitance of the capacitor. So, you can select a specific frequency (remember that a LC circuit has a natural resonant frequency). The device I found in the book works in a similar way. The theory is that the chemicals emit frequencies and those frequencies can be tuned. If you tune one chemical frequency and you detect activity on that frequency, it means that the chemical is present in your body. If you tune another frequency and you cannot detect nothing, it means that there is not such chemical present in your body or the amount of the chemical is extremely small so it cannot be detected with that device (maybe with another of a higher precision?).
The capacitor basically helps to block undesired frequencies and allow passing one frequency that we are interested in. It's like a filter. For that reason, you move the capacitor in analog radios. In this device would be something similar.
Machine learning is a field related with artificial intelligence. Basically, machine learning is used to discover and detect patterns within data. It's something like statistical discovery. You feed a computer with huge amounts of data, and the algorithms can detect patterns in that data. Maybe is you feed a computer with a bunch of unknow signals, the computer analyzes them and it can discover knowledge in that data. It's like computer vision... in CV basically you are feeding a software with numbers (from zero to 255) and the computer automatically recognizes shapes, colours, etc... with or without human help.
Some years ago Google performed an experiment where a computer learnt by itself, to learn what is a cat by seeing tons of youtube videos. No human ever said that there was a thing called a cat. The computer learnt to recognize itself, feeding the computer with lots of videos. That would be machine learning. It can be applied to anything that produces data. In our case, biological radio signals.
It's well known that when a disease or aging exists in the body, the electronic biological bandwidth gets narrow. Other researchers defend the idea that biology is about electricity and frequencies. When a disease is present, the frequencies varies because there is a change in chemical patterns.