Most PhD programs in the US (including CS, at least those I have any familiarity with) primarily admit students to a program; faculty advisors are chosen later.
These programs also tend to have extensive websites that explain the admissions process in detail, though you might have to dig a bit. For example the CS program at UW-Madison says about admissions decisions:
After careful review, the Graduate Admissions Committee recommends admission for the applicants they feel are most qualified for our program. The selection is made without regard to the degree goal (MS or PhD), area of interest or financial need. The department’s admissions recommendations are reviewed and approved by the Graduate School.
Nothing is mentioned about having selected an advisor or having their support ahead of time. If you dig further, they have a guidebook that mentions:
It is the responsibility of a PhD student to eventually find a dissertation advisor; the Department does not guarantee that a dissertation advisor will be provided. The dissertation advisor must be a full-time or affiliate faculty member of the Department, or have retired or resigned from such a position no more than a year ago.
"Eventually" - not before admission. Typically the first year is spent taking classes and beginning relationships with faculty members that could be potential advisors. I was not in CS, but my own grad program (neuroscience) had a more formal "rotation" structure, where new students spent the first couple of semesters in rotations with 2-4 faculty members, spending a bit of time in their labs to get to know them.
This is just one example - you will need to research individual programs to see how they do things exactly. I spent about 5 minutes finding this information for one program, though there's a lot more to read. You'll want to apply to several programs, and should plan to spend a good number of hours doing research like this into programs before you apply.
If you want to work with a specific person, I'd highly recommend reaching out to them in advance. What if you get admitted and then find out they are moving to another university? Or retiring? Or not taking students because they have too many, don't have funding, plan to go on sabbatical, etc. However, their blessing will not be required for application (unless the program says otherwise; do your homework) and may not even be particularly beneficial, depending on how admissions decisions are made.
Even if you don't have a specific person you want to work with, it can still be worthwhile to reach out, but not all faculty will be that responsive: you might just get referred to the application process if they'd prefer to not deal with prospective students until they are actually interviewed or admitted.