quantum mechanics - Question about the Double Slit Experiment with Single Particles/Photons

My question: In this version of the experiment, doesn't the single photon source's direction define which slit the particle passes through.

Also, with reference to the apparatus described here, where there is an additional single slit, does it mean to say that diffraction happens even for single photon's at the (first) single slit.

I understood what the experiment says after the particles pass through the slit, but I'm not able to understand how the single particle/photon source set up works.

Hope someone can help me understand how that works.