Criminal Investigations: Are Police Required to Forward Information to Prosecutors

My question involves police conduct in the State of: MI

Are police required to submit the results of their investigations to prosecutors for review once the investigations are complete, or do they have the discretion not to if they don't think charges are warranted for any variety of reasons (aside from cases where a complaint is completely scrapped because it is unfounded).

Also, why do police arrest first in some investigations and then wait to see if prosecutors will authorize charges, while in other cases they will wait until the prosecutor authorizes charges before arresting? Does this have to do with perceived immediate threat to public safety?

Criminal Investigations: Are Police Required to Forward Information to Prosecutors