Onboarding Super admin is different and it is an different URL
(1) https://onboarding8preview.sapsf.com, from which you can login into XpressHR Application
The above two URL are limited to ONB
Next :- Our Biz X provisioning
The Above 4 URL you have to coordinate and integrate into single on from Admin center.
Then you can start implementing ONBOARDING.
so that you can login to onboarding from your normal SF admin Center
SF super admin & Onboarding Super Admin are Different.
Let me know still you need any info.