The superuser implicitly belongs to the the administrators group: the core doesn't set it.
You can try it: if you login as admin and check the value of app(\Concrete\Core\User\User::class)->getUserGroups() you'll see that the group ids are 1 (GUEST_GROUP_ID) and 2 (REGISTERED_GROUP_ID), but not 3 (ADMIN_GROUP_ID).
The superuser implicitly belongs to the the administrators group: the core doesn't set it.
You can try it: if you login as
adminand check the value ofapp(\Concrete\Core\User\User::class)->getUserGroups()you'll see that the group ids are1(GUEST_GROUP_ID) and2(REGISTERED_GROUP_ID), but not3(ADMIN_GROUP_ID).