Like other psychologists, a PhD candidate works through conversation: an initial assessment, agreeing on goals and therapy using the method they are trained in. They are not physicians and do not prescribe medication.
In many systems therapists still in training work under supervision or consult a clinical supervisor. Whether a PhD candidate may practise independently where you live depends on that country's rules and on their earlier qualifications.
A brief comparison: a completed PhD means the research is finished and the degree awarded; a psychiatrist is a physician who can prescribe; a counsellor focuses on life and relationship issues. Asking a therapist about their degree, specialisation and experience is completely normal.