There are several security roles provided out of the box to enable you to get started quickly. You can edit these roles, copy and edit the roles, or create new custom roles from your environment's settings in the Power Platform admin center:
When you copy a role, you give it a name and all the access levels are copied for each privilege.
You cannot create new privileges yourself but when you create a custom entity, create, read, write, and other privileges are created for you. When you edit a security role you are restricted to just changing the access levels on each privilege.
To change an access level for a privilege, you simply click on the circular icon and it will rotate through the access levels from None to User to Business Unit to Parent: Child Business Units and then to Organization. If you click again, the access level will change to None.