What are 3 levels of OTC stocks?

Could you please explain to me the three different levels of OTC stocks? As a cryptocurrency and finance professional, I understand that OTC stands for Over-The-Counter and refers to stocks that are traded directly between buyers and sellers, bypassing traditional exchanges. However, I'm curious about the specific categorization of these stocks and how they differ from each other. Are there any key characteristics or requirements that distinguish one level from another? I would appreciate a clear and concise breakdown of the three levels of OTC stocks.