Apple isn't transitioning their handheld lines to USB-C.

Their portable lines — laptops, laptop-sized pro tablet — all use USB-C for charging and peripherals compatibility. Their handheld lines — cell phones, earbuds, headphones, TV remotes — all charge with Lightning.

(The older end of the Beats product lines use Micro USB due to having been an Android ecosystem product prior to Apple's purchase; however, Beats products released this year seem to be Lightning.)

I treat the non-pro iPad as a "handheld" device, and the pro iPad as a "portable" device. There are probably more nuanced words in use at Apple for deciding whether a product gets USB-C or Lightning, but I'm not invested in my choice of words, just in highlighting the distinction between the two categories.