Hello and hope you are doing well.
Most common cause for white patches is candidiaisis. It is usually caused by candida albicnas, which is a fungal infection. The white patches have a cottage cheese appearance. They can occur as a consequence to throat infection or sometimes steroid inhalers are known to promote their growth. You need to consult a doctor for proper assessment. In the meantime you can try warm salt water gargles and betadine mouth washes.
Hope this helped and do keep us posted.