if an employee brings terms of fair treatment to an employer or they have to quit, and the employer denies to treat them fairly and says he is agreeing to them leaving. Is the separation from the employer is called, “firing” or “quitting”, what is it called?

I think the term you are looking for is involuntary resignation.

You have to resign from your position due to the unreasonable work environment. But ya know, double check with lawyers and your governments definition if you are looking for something concrete