What's an Executive Order? Where is it spelled out what the do and how they're regulated? Can they be vetoed by another arm of government?

In the United States, an executive order is a directive by the president of the United States that manages operations of the federal government. [...] Like both legislative statutes and the regulations promulgated by government agencies, executive orders are subject to judicial review and may be overturned if the orders lack support by statute or the Constitution. Some policy initiatives require approval by the legislative branch, but executive orders have significant influence over the internal affairs of government, deciding how and to what degree legislation will be enforced, dealing with emergencies, waging wars, and in general fine-tuning policy choices in the implementation of broad statutes. As the head of state and head of government of the United States, as well as commander-in-chief of the United States Armed Forces, only the president of the United States can issue an executive order.

https://en.wikipedia.org/wiki/Executive_order