Hi Matt,
When a visitor visits the website for the first time where AAM is implemented. AAM assigns it a device ID called demdex ID.
AAM has out of the box integration with most of the famous ad platforms, ad servers, DSPs, data providers etc.
So, when a visitor comes on the website for the first time, AAM initiates a partner ID sync call, and the partner ID is assigned to that device. AAM will assign demdex ID as 1234, but an ad server may identify the same device with it's own ID: ABCD.
So, AAM syncs its own ID with that of partner, and maintains a match table. This process is automated, it does not require any set up or implementation change.
When you activate segments to destination, AAM sends partner IDs to that ad server. That's why you see different segment addressable audience for different destinations.
In order to check this ID sync process, you can open any website in a new incognito window of chrome and look at the network requests, filter it by ibs: and you will see partner ID sync calls.

Thanks,
Varun Kalra