It is clear from your comments that most of your knowledge about police work comes from TV and movies. Serpico was based on events that occurred in the 60's. Cops "rat each other out" all the time, they just don't call press conferences when they do so. Many complaints come from the inside, and yes, I have reported other officers misconduct. Nobody wants a bad cop gone more than the good cops he or she works with.
Most are taken off the streets but not fired immediately for various reasons, often strategic. Internal investigations generally don't commence until the criminal investigation is concluded, again for strategic reasons. Firing an officer immediately impedes the prosecution. You cannot compel a fired officer.read to answer questions or cooperate in an investigation.
Lots of people your age call the police, and the majority report that they were treated fairly and with respect. That includes people who got tickets or were arrested. Maybe most people are not as paranoid as you. news.gallup.com
Whenever possible, mentally ill detainees are released to a family member or voluntarily admitted to a treatment facility. If they meet criteria for involuntary examination (danger to self or others) then they go to the same facility with a 72 hour hold.
Sometimes there is no option but jail, for example if the victim of a crime wants charges filed, or if the person does not meet criteria for an involuntary hold and remains disruptive. Sometimes the person WANTS to go to jail and will brazenly commit a crime in front of an officer to force his own arrest. We call that a a 3HC arrest, (3 hots and a cot) Happens a lot with the homeless on cold nights.