To add Survey123 in app launcher on portal website, it does require extra steps to add item first with your enterprise/portal url.
Here is the step by step to add item first and then add app into app launcher:
Ypu can reference these online docs for a general steps to add apps on the Portal site here:
You might want to double check if your organization has licensed and if it does, it should automatically show up in the app launcher for user accounts which have appropriate licenses to access.
It's also possible that Survey123 maybe remove but you can add it back by following this doc.
As you are the admin of the portal, could you go to Organization tab and click Licenses tab>Applications and see if you have Survey123 assigned for the account you are logging with?
Here is the online doc about the user type licensing when you upgrade enterprise.
This is what I see when I click 'Organization' -> 'Edit Settings' and then the 'General' tab on the left. The App Launcher does not include Survey123, but asks for an App Item URL.
This is what I see when I click 'Organization' -> 'Manage Licenses'