I've heard that people interested in pursuing a Ph.D. in the US contact potential Ph.D. advisors to ask if they have space available in their labs. Does this apply to undergraduates (students with just a BA/BS degree wanting to go for a Ph.D. program directly) as well? I am looking for Computer Science Ph.D. programs. I thought all I had to do was apply to the university directly and then wait to hear back on my acceptance/rejection like with undergrad college application. Any input would be helpful.
Going through the same process, the way I see it is that the "normal" way to do a PhD straight from undergrad is to take courses for the first year or two, and then find a formal advisor.
That being said, I've seen that if there is a very good fit, professors can accept individual students directly to their lab/group as well.
So it is not a requirement to contact potential advisors, but can definitely help if you are absolutely sure that you want to work with them.