Short verstion (TL;DR)
I have a microphone connected to my new MacBook via a splitter adapter (see the picture below), in which you can use the multifunction jack separating the input and output channel, in order to be able to use it not only for headsets, but also to connect audio input sources.
The system says there is an "external Microphone / iPhone headset" device, and I can hear the mic if I test it when I connect it using the splitter, so I guess it detects it.
But it is not listed among the audio input devices, so there is no way to control it, that is using it as input in applications (Skype, Garageband) or changing its volume.
Do you know a way to make this work?
Long version (all details)
Based on the accepted answer to this question I have connected my microphone to my new MacBook via its multifunction combo audio jack, using a 3.5mm 4 Position to 2x 3 Position 3.5mm Headset Splitter Adapter - M/F (see the picture below). The purpose of this adapter is to split the audio channels connected usually to a headset, separating the input and the output, in order to be able to connect a microphone or an audio source just to the input channel.
It works and the systems seems to detect it. In particular, I can hear the microphone on the speakers/headphones. Plus, if I click on "About this Mac" and afterwards on the "Overview" tab I click on "System report", I see a window where if I click on Audio I can also see an item called "External Microphone / iPhone headset", which is the mic channel of the headset, which is what is detected when I connect the Mic like this.
A screenshot of this is below (see the red arrow).
What does not work
But when it is detected like this, the systems seems to not consider it as a proper microphone or audio input, and because of this you cannot control it, that is selecting it as source in applications like GarageBand/Skype or controlling its volume.
In fact, when you want to select the audio source for an application like GarageBand or Skype, it just doesn't appear, and I only see "built-in microphone" in the menu, while I would expect to also see the "External Microphone / iPhone headset) which appears if I click on "System report" as explained in the "what works" section.
Plus, perhaps because of this problem, I cannot even control its volume, which is quite low.
The system seems to just recognize that it is connected as part of a headset (because I connected it using the splitter above, again see this question for details), but nothing more.
Do you know any advanced setting (or terminal tool) for MacOS or any external application capable of controlling an audio input device (mic or musical instrument) connected like this, in particular setting it as the primary audio input or controlling its volume?
Alternatively, do you know any software capable of making the OS see this audio input device as a normal device instead of seeing it just as part of a headset?