Wondering if one could just outline what it takes to build such a realistic guitar, drum, or voice tool purely from scratch from basic components. What does it take? (Other than obviously probably a lot of work)? Is it simply a complex wiring/system of oscillators, or is it more like a physics engine for sound? If so, where can I find a reference to such an open source engine for inspiration? What do they entail at a high level -- the key pieces to create such realistic sounds? Not looking for an in depth exposition on such a broad topic, but simply an introduction to how it works, and where I can find inspiration on how to build one.
The key point to this question is I don't want to use samples of real sounds to build up a digital instrument. I want to create some sort of acoustic environment where sounds can be created, but my imagination is not quite equipped in knowing what is possible or how people typically go about doing this.