This get’s so confusing! So here is the simplified answer.
A psychiatrist is a medical doctor, and therefor their services are mostly covered by OHIP. A Psychiatrist can prescribe psychiatric medication, clinically diagnose a patient (i.e. assess whether a patient fits the diagnosis of depression, anxiety, or another mental health illness) and sometimes they will also offer psychotherapy.
A psychologist usually has a PhD in Psychology and is covered by (most) insurance companies in Ontario. They can clinically diagnose a client, but cannot prescribe psychiatric medication. A Psychologist usually offers psychological assessments and psychotherapy.
A psychotherapist may be covered by personal insurance benefits. They treat people using different psychotherapeutic techniques.
A social worker can also use psychotherapeutic techniques to help a client and use a biopsychosocial approach in their sessions.