How is this done in living brains? Presumably you have to vivisect sufficiently be able to shine light on neurons, or have some sort of probe that does it. At that point how is it easier than electrically stimulating the nerves to generate action potentials?

(To be clear, since it's probably necessary, I'm asking a curiosity-driven question about the mechanism here, not throwing shade on the work. Given it's won the researchers a Nobel prize it's obviously highly significant.)

Yes you put in a "probe" (fiber optic), just like when you stimulate with an electrode. The difference is electricity stimulates everything, while the light can be targeted, not by what it illuminates but what responds* because...

Ahead of time, you either use viral tools or other genetic modifications to make specific cell types express the photorecrptor.

*there is an effect of light/heat that all cells are subject to, and good experiments try to control for this as best they can