About 2D convolutions and how they produce a 1 channel image

Question

Trying to understand 2D convolutions, I ran into the following image, which has me confused:

If I understood correctly:

• the blue shape is the input
• the orange shape is the one of the convolution filters
• the green shape is the output

My question is: what are the calculations performed to get, from 2 tensors with shape 3x3xD (where D is the depth), a single value.

As far as I understand, the calculation of convolution would produce a 1x1xD vector, but I don't get how from this vector we get a single value. Is it just addition? Does it have normalization for the addition?

Thank you in advance!