First of all: I not talking about something philosophical. I am talking about what they techniclly are. Nothing else.
They are not clones with robot parts or something like that. That is just a common misconception. Clones are usally either grown using existing DNA to make a "copy" of some organism or some crazy machine copies someone (purely in fiction so far). Shaun's DNA is merely used as the basis to develop the synths but they are not modified clones of him.
Synths are (mostly) organic robots/ androids made to look and act like humans. They are assembled like any other robot but instead of metal and plastic components organic components are mostly used. They start out at peak physical condition.
It is also suggested that they don't need sleep or food and that they would stay the same (their body at least) throughout their entire existence without outside interference which makes them a bit more roboty in my opinion but the same could maybe be achived through technology in the Fallout universe.
I thought it would be nice to clarify what synths are. If you find language related errors then you can keep them.