Well, there’s the right way to do it (which is VERY expensive), and there’s the way I’ll be doing it.
The equipment I need is fairly minimal - a solder paste stencil, a temperature controlled hot plate, some solder paste, and a squeegee.
The stencil is exactly like a silk-screen stencil, except it is very precisely laser-cut into a sheet of Kapton (a high-temperature plastic) or stainless steel. My templates are stainless. It has cutouts everywhere solder needs to be applied. The solder used is a paste, a mixture of powdered solder and flux. The stencil is laid over the board, and precisely aligned so the cutouts all line up with the pads that need to get coated. Then a squeegee is used to spread the solder paste over the stencil, and push it into all the little cutouts. Then the stencil is carefully peeled off, leaving the solder on the pads on the board.
Then, the components are carefully put in their correct places on the board, and the board is placed on the hotplate.
The hotplate is heated and cooled through a specific cycle of several temperatures to heat it up to 120C, avoiding thermal shock (which can damage the board and components, and create residual stresses, particularly on the solder joints), then heat it further to 150C, melting the flux, then heat it further to 180C, melting the solder, letting it soak long enough at 180C for the solder to flow completely, then cooling it down to 100C slowly, again avoiding thermal shock.
My PCB “panels” are small (100mm x 100mm), each containing from 6-8 individual PCBs, and I should be able to fit four of them on the hotplate at one time. So, I can, in theory, solder up to 32 PCBs in one run. It is slow, tedious work, but this process is well suited to the low volumes I would be producing.
In a production facility, the ENTIRE process would be fully automated.