Each school is different. Some schools (like Duke) for example uses a 2 reader system. After 2 reads (and sometimes 3), the scores are tabulated. Depending on the score, an applicant can be auto-admit or auto-deny. The rest go through committee evaluation.
Committees meet regularly, usually once a week and make decisions on a rolling basis. At the end, everything rolls up to the Dean of Admissions. The Dean then ‘sculpts’ the class based on desired characteristics. Some students are removed from the accept pile, and others are added. Once final sculpting is done, the decision letters are posted online.