Maybe not following, but I buy strips of WS2812 and if I wanted to for them to turn on and, say, display a rainbow, I need something to drive that. Not USB from another device, e.g. standalone.

Yes but as far as I understand the OP is not doing that, they are just using raw LEDs with power drivers and the whole shebang that is so nicely built into the ws2812s for us. Otherwise they wouldn't need these components they're talking about.

I just connect them to a microcontroller pin and be done with it. I power them separately off a power bank (my LEDs are almost always worn, if they are static I just use an off-the-shelf 5V supply).