Who actually composes the AA ?
In this case, the short answer is Corporal Bret Payne of the Moscow PD drafted/signed the subject AA.
Generally, the district attorney, representing the people of the state, makes the decision whether or not to bring charges and/or prosecute a defendant in one of two ways -- a grand jury indictment or by Information and Complaint by the investigating police department.
The prosecutor comes about the police department's criminal investigation information from the arrest warrant affidavit/probable cause affidavit (i.e., AA or PCA) attached or an exhibit to the police department's application for an arrest warrant presented to the Judge for signature, followed by the execution of the warrant by police.
Here, we know that several agencies including the Idaho State Police, Coroner, FBI, WA and PA authorities, and the Latah District Attorney worked together with the Moscow Police Department to investigate the murders, and the contributions by these partners were ultimately included in the 19-page affidavit by CPL Payne dated December 29, 2022. MOO Ah..the long answer.
(For the latter example of a DA filing homicide charges without a police AA, more recently, those following PA's missing/murdered Elizabeth Capaldi might recall the weeks we spent wondering what in the world Bucks County Sheriff/Police were doing to investigate the missing wife/mother when suddenly, on December 15, Bucks County District Attorney Matt Weintraub announced the husband (Steve) was arrested, and his office filing homicide charges after he was indicted by the grand jury (secret) for murder).