They see him as the absolute Wali Faqih, while absolute authority only belongs to the prophet and 12 imams.
My other question is whether or not the Supreme Leader’s actions can be monitored or not? In the case of any violations, who is to be referred to?
The answers to the first and second part of your question can be found in the following links: Question 920 (website: 2975) (The Wali Faqih and Infallibility) and Question 786 (website: 845) (Determining the Wali Faqih) and Question 3105 (website: 3370) (The Scope of Wilayat Mutlaqah Faqih).
As for the third part of your question about the supervision of the Supreme Leader and who to go to in the case of any violations, we must say that the Assembly of Experts, who are usually pious and unafraid individuals and are voted for by the people, are the ones responsible for monitoring the Leader’s actions precisely; in the case of any violations, it is their religious and lawful duty to warn him as advice for the leader of the Muslims. Also, concerned people can make their voices and suggestions be heard via letters and his official websites. If at any time, the Wali Faqih loses any of the conditions for being Supreme Leader, in addition to automatically losing his position(in Allah’s eyes), it is the duty of the Assembly of Experts to remove him from his post and appoint another qualified individual to take his place.