@NataliePortland

It's a "trimmer" (like a router but smaller, thus less expensive) to make a "canal" where to place a LED strip.

He probably sketches a line to follow that he can see through the bottom of the router. It's probably around 3/4 inch bit, it just needs to be wide enough for the LED rope, so maybe 1 inch. It would take a couple passes to get that depth, but the first would be the hardest.

@NataliePortland routing ?

That's an interesting question. I don't know how that's done, and conveniently the captions all videos have to have nowadays are in the way of what I need to see. I don't know how he stays on course.