As an admin, you always have access to the wallet that holds all the user funds. This is a custodial solution.
The plugin maintains a ledger of transactions (deposits, withdrawals, internal transfers between users, and exchange trades) on the MySQL tables, and it derives the user balances from summing these transactions.
You can always edit these transactions and thus modify user balances. With wallets5, this can be done via the MySQL tables, unless if you want to do something simple, like cancel a transaction. In the upcoming wallets6 plugin, it will be easier to edit transactions via the admin interface, because transactions will be editable post types.
Hope this answers your question. Let me know if not or if you have any more questions.
P.S. I moved your question to the general discussion since it was not related to the other thread.