Custom attributes from on-prem AD (by extend schema) can be sync'ed to AAD:
Azure AD Connect sync: Directory extensions - Microsoft Entra | Microsoft Learn
How to sync Custom Active Directory Attributes to Azure AD? (rebeladmin.com)
Sync'ed attributes can be retrieved thru a pre-built app called "Tenant Schema Extension App"
This is not the same as "external identities | custom attributes", or "B2C | user attributes", but using same mechanism behind the scene.
In fact, using same mechanism, you can just "extend" AAD to have custom attributes without the help of ADC or external ID or B2C.
Azure AD Graph API Directory Schema Extensions | Microsoft Learn
Please note, it's not a true extension of AAD schema (MS won't allow that), but a clever way of utilizing an app to store such "extended attributes".
This is also different from "custom security attributes". Not sure if "custom security attributes" is using same technique (likely not)
Other reads/Best summary:
Custom security attributes in Azure AD part 1: a trip down memory lane | Blog (michev.info)
See also specific case of syncing on premises AD extend attributes to AAD (extend attributes in this context is any attribute that is not included by default by ADC engine)
Sync additional attributes from on-prem AD to AAD