They were just public listed apps. You’d have a corporate app on the normal store which would just hit a login wall. Not really that big of a deal but not as nice as an unlisted app.
There's also Custom Apps[0] where you go through the same process but can either generate redemption codes to send people or go through the Mobile Device Management tool. These are not on the public app store.
I setup this for a company app that their employees use.