Is your deposit in rubles or in a foreign currency? Because that matters.
So yeah, they do take 13% from the interest you earned, not from the whole deposit amount. The bank usually withholds it itself and transfers it to the tax authority. But here's the catch: if your deposit is in a foreign currency, the tax is also calculated at the exchange rate at the time you receive the interest - so you might end up paying more than you expected if the rate changes. Plus there's a tax break - if the balance in your account doesn't exceed a certain limit, you might not pay tax at all, but honestly, that's rare these days.