I didn't have any code on this part. It's all the fresh-out-of-the-box Godot stuff that goes on behind the scenes.
It's just like when you stick a bunch of UI elements into a brand new scene and have it grab_focus()
Buttons automagically find neighbors and stuff, and the controller switches between them with the default input map.
I just did this with some sliders. By default it lets me adjust the values of the sliders with the joypad, but only by one tick per joystick press. I was trying to keep the code additions to a minimum. If there's no "built in press-and-hold" mechanism I'll probably just have other buttons that adjust the sliders by, say, 10 ticks. But I was hoping there was some other "out of the box" code, or line of code, that would accomplish this.