Generally speaking, you can't. That's just not how SQL works. SQL is based on querying one TABLE at a time. There may be interesting ways that you can list out your tables and do some sort of join or subquery with it, but even then your search process is going to be anything BUT efficient.
Even then, that won't address your data being in multiple databases (and I use the term "database" with Access very liberally).
The "proper" way to go about this is to merge your data into ONE database, and if possible, into ONE table. Then, index your data so it can be queried efficiently. After that, your searches on such a huge amount of data may even perform respectably in Access.
Wish I had better news for you, but that's just how SQL works.