I would advise to typin the commands to practice than using Enterprize Manager. That way you know what you are doing and syntax also rather just select and click in OEM to get the task done.

SYSMAN is superuser for OEM like sys and system oracle database.

If you login as XYZ, and you do have permissions to create table and tables you create will be under the ownership of XYZ. Obviously to create table user needs to 'CREATE TABLE' privilege which you have to grant when you create user 'XYZ'.

You create users XYZ and ABC and give login privileges to Enterprize Manager. You can login as XYZ?ABC as long as you know the password for those users.