It could be a pretty long answer so the best way if you're going to work with EV alot is to go on one of the courses.
But in short.
1.EV server starts processing.
2. Via the outlook client it will connect to Exchange via MAPI for each mailbox to be processed.
3. Scans through each mailbox, any mail matching the archiving criteria will be archived
4. The mail is copied to the storage queue on the EV Server.
5. Mail get stored in the vault store partition and vault store database updated with the meta data of the archived mail file/files.
6. Once backup occur on the vault store partition, EV will replace the mails in the mailbox with a shortcut.
It's more to it, e.g. single instancing etc.
But again, if you want all the details about the MSMQ-queues, databases etc. then you're probably looking for the admin courses for EV.