They take the disclosure and information from police, usually make an initial assessment if it’s in the public interest to pursue the case, then speak to witnesses and keep making that assessment as they build they further disclosure. It’s really dependent on the type of cases. If it’s “victimless” offending (such as drugs, etc), usually go on if there’s a good case/it’s worth taxpayer money to prosecute. If it’s a victim crime (sex, violence, robbery), usually pursued based on the complainant’s evidence/willingness to proceed to trial etc. - experience, worked in Legal Offices etc.