Can one person be the product owner and the scrum master?
The answer to that question is no, this is not recommended. These two roles have significantly different scope and are ideally fulfilled by two different people. Often in practice the SM needs to protect the team from the demands of the PO while the PO needs to maximize the value the team delivers - this is another reason to split the roles to different people. If one person tries to fulfill both roles, that will not only be difficult but may also lead to natural conflict and be impractical.