what if I don't "look American"(non-white, non-black, for example)
and I answer "YES" to the Border Patrol agent's question? If he/she
does not believe me, will I be detained and sent to immigration detention
center?

why do I get the feeling that the DHS is trying to find a way to make
every US citizen carry papers at all times? (like the former USSR or
most countries in the EU today)

also, by setting up interior immigration-status checkpoints, isn't the
Border Patrol basically admitting that they have already failed to secure
the physical US/Canada/Mexico border in the first place?