Flow Builder uses an Apex action to provide additional information about multiple Contacts, stored in...

@InvocableMethod(label='Additional Info') public ContactInfo getInfo(Id contactId) { /* implementation */ }

@InvocableMethod(label='Additional Info') public static List getInfo(List contactIds) { /* implementation */ }

@InvocableMethod(label='Additional Info') public static ContactInfo getInfo(Id contactId) { /* implementation */ }

@InvocableMethod(label='Additional Info') public List getInfo(List contactIds) { /* implementation */ }