EDAS has several privileged roles: conference chairs (usually just referred as chairs), publication/registration chairs (there is only one designation for both roles), travel grant jury chairs, track chairs and TPC group leaders. Track chairs and TPC group leaders only have review-related privileges (assigning and accessing reviews), while chairs and publication chairs have wider responsibilities, such as configuring the conference, inviting TPC members or creating conference proceedings.
Conference chairs have almost complete freedom in running their conference. They can add and delete TPC members, change submission deadlines, add tracks and change review questions, upload papers after the deadline, or change paper information, for example. They have all the privileges of publication chairs, travel grant chairs, track chairs and TPC group leaders.
Some chair priviles can be configured via Conference:Configure, "Roles & permissions".
Society staff can also be treated as chairs for all conferences and journals sponsored by that society, without having to explicitly add them as chairs.
For their own papers, a chair is treated just like any author, e.g., they cannot see reviewer identities of papers they have co-authored and cannot see review and scores until they have been made available to authors. However, if a chair wants to "cheat", there are probably ways to find out information, e.g., through some of the paper listing functions. The feature is mostly meant to prevent accidental disclosure of information, not willful snooping.
You can set up conferences as master and subconferences, the chair of the masterconference is treated as if he or she was a chair of the subconference as well.