The Marketing Cloud ID service maintains your existing Analytics IDs by migrating them into the new AMCV cookie as an additional key value pair. The legacy AID is used as the primary key in reporting for that visitor with the mid being a secondary key.
Jantzen - Thank you for the resources. I have two questions regarding the video:
(1) Why the ID service code has to check the demdex.net cookie for the UUID and then use it along with the OrgID to generate the MID, INSTEAD OF simply getting the MID directly from the first-party domain?
(2) In the scenario of SiteB, the final codes set on SiteB.com domain should not include the AID because the visitor didn't visit SiteB before (and thus had no legacy s_vi cookie), right?