There are two rounds for admittance to the Commerce School during your second year. The Comm School posts detailed info on their website about the challenges of admittance.
Some people decide to attend another university that has a business school that is direct admit from high school because they don’t want to take the risk of not getting in. However, there are also many students at UVa who start out intending to go to the Comm School, and then develop other interests in another major.
Students in other majors can take a 3 intro Comm School classes even if they are not part of the Comm School.
A number of UVa bachelors degree recipients enter McIntire’s masters degrees programs. You are allowed to enter those programs straight after your bachelors degree, unlike Darden, which requires years of experience before applying.