This is a very broad question. How the cryptography operates depends completely depends on the type of encryption used (a specially secure DB, a DB file encrypted, encryption using special SQL constructs or application based encryption. etc.). Then it also depends on the key system used, public key or secret key encryption and how the key management and access conditions are setup.
If you take a birds eye view then encryption doesn't make sense: to decrypt (and, for symmetric ciphers, also to encrypt) you need access to a secret key. So instead of securing your data you now have to secure a key. This key may however be generated in advance and is much smaller than the data. But more importantly it may have a different protection and may be restricted in access.
The more protected and least accessible the key is the more secure it is. This comes at a price of course. Say you'd use a passphrase or a private key on a special device. In that case you need to supply the key during booting of the DB, and you may need to provide continuous access to it without breaking the security measures.
So onto your questions:
How I can use an encryption model so that this encrypted data (user/pass/token) can be seen by an admin, but not seen (encrypted) by others?
By encrypting the data in such a way that it only can be decrypted by a private or secret key held by the admin. Sometimes a data key is e.g. encrypted with the key of the admin, and that data key is in turn used to encrypt the data in the database (so you get a hierarchical structure where the security of one key depends on another, higher level key).
I'm not sure I understand. Where is the encryption key being stored, and how is access gained to it? What is the encrypted data and its relation to user/pass/token?
Where the key is stored depends on the system. Hopefully it is stored in some kind of secure store, e.g. protected by the operating system. But it may also be a static key in the application code or stored in a configuration file.
Banks and such tend to store the most sensitive keys in HSM's (if the keys need to be directly accessible) or off-line (on an offline storage device that can be used by a hardened laptop). On smart cards and HSM's the keys can only be used, not retrieved (exported) except for back up purposes. Using the keys generally requires the user or system to authenticate to the storage device.
But books have been written about key management, and in the end you may need to read a few to fully understand how to handle keys.