As a US citizen with a PhD, I didn't experience any clear discrimination in favor of foreign students during grad school.
I think the main reason so few US citizens get PhDs is because PhD "student" (they're actually workers) positions pay so poorly. Make PhD student positions have non-poverty wages and you'll see a lot more interest from US citizens.
On the flip side, I think foreign students experienced a lot of abusive conditions that I could more easily say no to because I didn't have a visa that required me to work at the university. I've seen some of that first hand. I don't mean to imply that there would be no cost to me saying no, just that I wouldn't have to leave the country if I said no.
im referring to the admissions process, and this discrimination has been present for decades
I've seen clear discrimination in favor of foreign students, but it was specifically because of those abusive conditions. I know of professors who exclusively tried to recruit specific foreign nationalities (their own, typically) because they could get away with treating them worse than American students. I wouldn't have been able to get into those labs, but I also wouldn't want to.