I know the symptoms and what schizophrenia and bipolar disorder are but in what way are they related to substance use?

Not always related: Schizophrenia, bipolar disorder and substance use are not always related. Patients with these illnesses will sometimes self medicate with substances to help them cope with their symptoms. Patients with no prior mental illness will find themselves with schizophrenia-like symptoms due to drug abuse. There are patients with these illnesses that do not use drugs. Tobacco use though is very common.

Drugs/etoh = problem: If you have bipolar disorder and abuse drugs or alcohol - you are much more likely to develop mixed, irritable states vice euphoric manic states. Drug or alcohol abuse really minimizes chances for success with treatment. Although people who are suffering may self-medicate with drugs / alcohol - it is likely to cause their symptoms to worse.