Actually, the CoinPayments wallets are used to back the wallets of all the users, not just the administrator. All the user funds are placed in one wallet. The plugin retains information on who owns what so it can display user balances and allow withdrawals. There is a 1-to-N relation between actual wallets and users. This is the case with all coin adapters. Coin adapters allow the plugin and its app extensions to communicate in a uniform way with any backend wallets.
With the Fiat Coin Adapter, there is no wallet, hence no wallet API to communicate with. The administrator is responsible to actually carry out bank deposits and withdrawals, and to record these changes to the plugin, with the help of the provided tools. When a user requests a withdrawal, the administrator must actually carry it out and mark it as done in the plugin. Similarly, when a user performs a deposit, the administrator is responsible to check that the deposit was performed to the site’s bank account, and then to mark the user’s deposit as done.
Hope this is a bit clearer. If you like to learn more about the plugin you can have a look at the glossary section of the plugin’s documentation where all the concepts are explained.
Please let me know if you have any more questions.