Is it standard for US-based universities to consider the ethnicity of an applicant during PhD admissions?
If there is one thing that academia is particularly bad at, it is transparency in decision making. Why do some people get admitted and not others? Why are some people hired and not others? Why are some people promoted and not others? From the bottom to the top of academic hierarchy, you'll be hard-pressed to ever get a straight answer, and answers you do get are always suspect because the nature of human decision-making is itself complex and opaque. There are plenty of reasons for this, from self-protection from criticism and gaming of the system, to rapidly changing criteria based on past experience and argument, to the fact that so much of the decision making is based on subjective ratings provided by individuals (often down to individual faculty members deciding if they want to work with a student).
To cut to the heart of your question, while it is not any sort of prescribed standard at the graduate level, you can safely assume race, ethnicity, national origin, and gender will be in some way be "taken into account" by some committees and individual professors. In many states this is actually illegal, explicitly forbidden by law - you can still assume it will sometimes happen anyway. On the other hand, some people will simply ignore demographic factors to the extent they are physically able, regardless of what they are told to do. As Universities have a history of avoiding transparency in admissions, you are extremely unlikely to ever know when it was considered, how much effect it had, whether it was positive or negative, and whether it ended up materially effecting the decision to admit you or not. You cannot know, they will basically never tell you, they will tend to go out of their way to avoid writing it down, and it is common for many people in the process to not even know with any certainty what factors ultimately "mattered".
However, from what I've seen on the inside and the outside of academia and from talking to fellow academics, you can safely assume at the graduate level that other factors matter way, way more. It is simply too high-stakes a decision for most schools to be altruistic, and they tend to steadfastly refuse to intentionally admit people who are utterly unqualified simply because of some demographic fact about them. The vast majority of Universities will only admit you if they think you will do well there, because having a bunch of under-performing students who are radically unqualified to pursue research and teaching is an albatross around the neck that few graduate schools will willingly tolerate. This is even more true at the PhD level, because they aren't cashing in on tuition (which is heavily marked up for international students, because money), so performance is king.
From talking to people working on diversity initiatives, I'll also give you another important insight: the diversity initiatives you hear about are not actually a preoccupation of the majority of faculty and staff, for good or ill. According to people working on actually making these initiatives successful, they report that the vast majority of people mostly want to ignore the issue and focus on other things, and so they work so very hard to make the initiatives public and heavily seen because otherwise they feel they will be unable to make any changes and they will be crushed by the majority weight of indifference mixed in with a heavy dose of active hostility to their efforts. Higher ed can look monolithic from the outside - but on the inside it is highly factious, and what appears as consensus is often a thin facade.
Because of all the above, the only advice I think makes any sense is treat the process as any other student should: apply broadly, see what offers you get, and talk with prospective advisors about what they are interested in. You are welcome to ask them if they were involved in your admission decision and then ask them why they wanted to admit you; I did that, and while it can feel a little awkward at first, a little bit of humor makes it easier and most professors when questioned directly, sincerely, and politely gave very direct answers. Some even said what parts of my profile they thought were a concern, and what other parts they thought would be a strength. Everyone gave wildly different answers, there was no consistency - no two people see the same person the same way.
Once you've got all your offers and finished talking with people, you can make your decision then on what seems like the best opportunity for you.
In addition to underrepresented racial and gender groups, some other diversity goals that many universities often care about are geographic diversity (not everyone coming from NY and CA) and admitting more first-generation college students.
I want to first echo what everyone else said, which is that schools don't admit people who they don't think will succeed, and if you're admitted they think you're a good fit. Second, the idea that if you put race and gender considerations aside then the default would be some kind of "pure meritocracy" is a fiction. There's a huge amount of randomness in students life experience and opportunities, in how good your letter writers are in writing letters, in who happened to be in the admissions committee that year and which letter writers they know. Schools also might want to balance between fields, and so if AI happens to get too many applicants at some school some year it might be harder for you than for someone in a different subfield. (And at the undergraduate level, there's a huge advantage given to legacies and athletes in obscure sports, both of which skew very rich and white.) At the graduate level probably the biggest of all, is there's a huge advantage given at state schools to American citizens and permanent residents (whose tuition waivers are significantly cheaper for the department to pay). If you're not a permanent resident, the disadvantage that causes is going to wildly outweigh any advantages that diversity considerations could possibly give.
Finally, I want to say that there are lots of simple practical reasons that diversity in hiring and admissions is practically important for departments. One of the most important ways that we are evaluated by the school is in attracting and retaining majors in our degree. On the whole, students are less likely to pick a major if they don't see any faculty or TFs with similar backgrounds to them. The African-American faculty I know are inundated with students who want to meet with them so that they can talk to a faculty person who understands their experience. Departments who don't have enough women faculty or don't have any African-American faculty are unable to do as good of a job of meeting the practical needs of their students.
I'll answer as far as how I perceived this working in the PhD program I graduated from, which was not in CS/AI.
A) No one was admitted who was unlikely to succeed in the program. No one was admitted who didn't belong here on merit in order to fill some sort of quota. No token admissions.
B) In every applying class, there were more applicants that the program wanted to admit than the program had funding to accommodate. Some of those students could be supported directly by a lab and join even if the program was out of money, but they would not have a year to rotate and test out different labs for a good fit.
C) Some additional funding was available to students from under-represented groups, however, only available to US citizens/permanent residents. Funding sources for international students were more limited, and they typically came either with funding from their home country or joined a lab directly that funded them.
In summary, the main way that ethnicity could matter was by funding for a student to join the program as a fellow/trainee rather than directly funded by a lab, which would give an opportunity to rotate in labs for a year that they might not have had otherwise. This was only even relevant for students who were already desired by the program on merit. As an international student this wouldn't have applied to you. Other programs may have different approaches that apply to international as well as domestic students.