Are shader passes drawn by object or by triangle in Unity?

Question

In Unity, when we write a custom shader with multiple passes, are they executed:

For each triangle do:
    For each pass do:
        Draw the pass

Or:

For each Pass do:
    For each triangle do:
        Draw the pass

And if I have multiple materials in the mesh, are the faces drawn grouped by material?

Answer 1

Question 1

Second variant: In Unity, when we write a custom shader with multiple passes, are they executed:

For each Pass do:
    For each triangle do:
        Draw the pass

Question 2

And if I have multiple materials in the mesh, are the faces drawn grouped by material?

Yes.

Explanation

Basically, all the rendering (in both OpenGL and Direct3D) is done following way:

1. Setup rendering parameters (shaders, matrices, buffers, textures, etc.);
2. Draw a group of primitives (this is called draw call);
3. Repeat these steps for all the graphics that needs to be rendered.

And the following heuristic rule is applicable: the less draw calls invoked in scene the better (for performance). Thus, in order to fulfill this rule (without reducing amount of primitives you draw) you want to have smaller number of draw calls with bigger number of primitives in each. Which in turn makes it obvious why Unity goes with second variant of your first question. And also explains why does Unity groups faces by material (because it minimizes draw calls number).