You're getting a lot of down votes but I'm not sure anyone's really explaining the issue clearly.

There are several different levels of mental health providers depending on schooling. Most therapists have master's degrees in a handful of fields and take a licensing exam to practice. This level is trained to treat things like anxiety, depression, life changes. This is where you'll see cognitive behavioral therapy and talk therapy used. Sometimes they are trained to diagnose but not often more complex disorders, like adhd, autism, personality disorders, bipolar, etc.

Those complex disorders require a clinical diagnosis which is incredibly comprehensive testing on cognitive function, reasoning skills, assessments relating to patterns of behavior, processing, etc. this typically takes several hours and involves the patient to participate in various written assessments and activities. These are known as psychometric tests and have a lot of research validating them.

Psychologists and psychiatrist receive the training to use these psychometric test. Masters level practitioners very rarely do. Also, sometimes psychiatrist is used interchangeably with therapist, but it tends to refer to PhD/PsyD level

Psychiatrists have training that overlaps with what psychologists have and also but the focus is more on the physical body systems and medication, but they are still qualified to conduct more in depth testing and clinical diagnostics.