How does the Marketing Cloud handle Lead conversion to a Contact

When a Lead Converts to a Contact, their SalesforceID will change but they're still the same person with the same information, same data, and same tracking so the Marketing Cloud will need to ensure we're still mapping all of that correctly.

ContactAlternateKeyStore (CAKS), is internal architecture and our way of handling Leads converting to Contacts. CAKS provides that extra layer to the Contacts Model be able to handle the Lead Converting to a Contact.

How CAKS works is, when you create a Lead, the ContactAlternateKey for the LeadID = ContactKey and the AlternateKeyValue and ContactKey are equal.

When you then convert the Lead to a Contact, the values change and the ContactID replaces the LeadID and this then update All Contacts (view in Contact Builder) too, to ensure the record transitions smoothly.

If you want to delete the converted Lead from the Marketing Cloud then you can refer to this article: Marketing Cloud Contact Delete on Lead synchronized data extension deletes converted contact

Email Send and IER details are not transferred to the contact when converting lead but the data is not deleted when we convert a lead and can be accessed by creating reports or by using a SOQL query.

Here is an example of a SOQL query that you can use to fetch the data of all Send definitions:
 

select et4ae5__EmailId__c,et4ae5__EmailName__c,et4ae5__Lead__c
from et4ae5__SendDefinition__c

Here is an example of a SOQL query that you can use to fetch the data of all Individual Email Results:
 

select et4ae5__Email__c,et4ae5__Contact_ID__c,et4ae5__Lead_ID__c
from et4ae5__IndividualEmailResult__c