How does OpenGL know what type each vertex buffer object is?

Question

I've just read through a tutorial about Vertex Array Objects and Vertex Buffer Objects, and I can't work out from the following code how OpenGL knows the first VBO (vertexBufferObjID[0]) represents vertex coordinates, and the second VBO (vertexBufferObjID[1]) represents colour data?

glGenBuffers(2, vertexBufferObjID);

// VBO for vertex data
glBindBuffer(GL_ARRAY_BUFFER, vertexBufferObjID[0]);
glBufferData(GL_ARRAY_BUFFER, 9*sizeof(GLfloat), vertices, GL_STATIC_DRAW);
glVertexAttribPointer((GLuint)0, 3, GL_FLOAT, GL_FALSE, 0, 0); 
glEnableVertexAttribArray(0);

// VBO for colour data
glBindBuffer(GL_ARRAY_BUFFER, vertexBufferObjID[1]);
glBufferData(GL_ARRAY_BUFFER, 9*sizeof(GLfloat), colours, GL_STATIC_DRAW);
glVertexAttribPointer((GLuint)1, 3, GL_FLOAT, GL_FALSE, 0, 0);
glEnableVertexAttribArray(1);

Edit: Thanks to Peter's answer, I found the following two lines of code which hook up each VBO with the shaders (indices 0 & 1 correlate to the VBO index):

glBindAttribLocation(programId, 0, "in_Position");
glBindAttribLocation(programId, 1, "in_Color");

Answer 1

It doesn't, you have to tell it which is which in the shader.

layout(location = 0) in vec3 position;
layout(location = 1) in vec3 color;

void main()
{
    /* ... */
}