Technically, by default there are no roles – it’s only the server founder/creator. However, the person who made the server can make roles and put whatever he wants as the highest.
Nominally server owner though they can give that up. Generally, the highest role is whatever you decide it to be.
the specific name of the role will change based on the server, but the owner/original creator of the server always has the highest role
It’s the Owner role. But then the Owner still has the highest power in a server even if there’s no Owner role that was previously created. So in that case, it’d be the Admin role. The Admin has the highest power only second to the Owner who has the permission to delete the server which the Admin can’t do.