List of Active Directory Attributes Mapping to Azure AD Attributes

SATYAM GUPTA The default and recommended approach is to keep the default attributes so a full GAL (Global Address List) can be constructed. Install Azure AD Connect with default attributes and see if you see all required attributes in GAL. Even if you choose all attributes to sync from ON-prem AD, Azure AD does not has all the attributes available from on-prem AD. for e.g. on-prem AD has an attribute called Employeetype which is not available in Azure AD. in that case you have to create the custom rule. etc

for now, just go with default and tune it according to your needs

https://docs.microsoft.com/en-us/azure/active-directory/connect/active-directory-aadconnectsync-attributes-synchronized#attributes-to-synchronize