Hi there,
Welcome to AAM community forum!
There are 2 types of integration between AAM and Campaign: Declared ID (customer id) and MID.
Declared ID is the hashed email IDs that we receive from Campaign and that campaign understands. In order for them to be understood by AAM, there should be ID sync implemented. For this integration, the data gets onboarded to a cross device data source in AAM, so if there are IDs those have not yet been ID synced, they will still retain in AAM. But they should be later on synced to be targetable.
The other type of integration is MID, that campaign and AAM both understand, provided there should be experience cloud ID service implemented.
For more information, please check this doc:
Audience Manager to Campaign Implementation Guide
Thanks,
Varun Kalra