The board is running JetPack 4.4 installed over Debian Buster. The nv_tegra_release says
R32 (release), REVISION: 4.3, GCID: 21589087, BOARD: t210ref, EABI: aarch64, DATE: Fri Jun 26 04:38:25 UTC 2020
SInce you ask… As I understand it, JPEG is essentially a 4:2:0 format, i.e. each 2x2 block of pixels has four Y, one U and one V value. My iOS code tells libjpeg to output YCbCr and it gives me Y, U and V values for every pixel, which looks like 4:4:4, i.e. libjpeg has upsampled. I then downsample by discarding alternate U and V values horizontally to get YU YV, which is 4:2:2 since there is no vertical downsampling. This is what the Apple YCrCb texture format wants. The result is 16 bits per pixel, with better quality than the alternative 16 bit choice, RGB 565. If there were a way to also downsample vertically it would be 12 bits per pixel with the same quality.
Anyway, this isn’t particularly important; I’m not currently limited by RAM so I can just use RGB for everything.