Quick guide on how the RBAC is currently set up on Exchange Online.
First thing is the management scope, this groups all mailboxes based on search filters into groups:
Once the management scopes are created, they will appear in the EAC under Roles > Admin Roles > Add Role Group:
You can also tick an existing group and copy the role to have the same permissions for the new group.
Set a name and description for the role and click next.
Here you can assign permissions to the role. You can copy an existing group to make things easier or reasearch what permissions you need for the role requirements you have.
Add users to the role and click next then finish.