PCBs, copper pours, ground planes, and you

Ok I'm gonna ask a potentially stupid question as someone who knows little to nothing about electronics. The article (and every other source I looked at when trying to answer my question) points out the blank spots on the board as a copper pour. Where, exactly, is the copper? I just see green plastic. Is it on the back side and nobody is taking photos of it?

A PCB is a layer cake of fiberglass and copper, with the green stuff just being paint on top to make manufacturing easier. The copper is glued to the fiberglass and etched according to your design files. (This turns a plate of copper into a bunch of very thin wires, basically.) If it's a multi-layer design, a bunch of these etched boards are glued together. Holes are then drilled for vias (which are rivet like things that electrically connect arbitrary layers according to your design file) and through holes (for through-hole components, or for screws to attach a board to an enclosure, etc). Finally, a solder mask (the green stuff) is applied over the parts of the copper that don't need to be exposed (for connecting components), covering the copper on the outside layers.

The solder mask prevents solder from sticking to parts of the board that don't need solder on them.

To add components, solder paste (little balls of metal embedded in a flux) is applied to the areas without solder mask (using a metal plate that is laser cut to have holes where solder is needed), and then an optical/robotic system called a pick and place machine places each component at the right spot. (Or you can do this with tweezers.) Finally, the board is heated to the melting point of the solder, and because of the solder mask, pulls all the components into the exactly right place (by minimizing surface tension, something liquids like to do). Through hole boards are a little different, they get "wave soldered" where the board floats on molten solder and attaches to the metal areas. The solder mask is even more helpful here. If you didn't have solder mask, you'd just get a thick layer of solder on top of every trace, and potentially bridges between adjacent traces, which is bad. (But people do apply solder to exposed traces to increase their current carrying capacity.)

TLDR, the plastic on top is paint that makes manufacturing easier. If you make PCBs in your own shop with a laser or mini CNC, you won't have solder mask, and you can see how much more difficult soldering components is. It's not impossible. Just a little bit more work from your hand and brain is required, and that's expensive at scale. So, plasticy paint.