With several debts at once the question is not whether to overpay but which one first. The snowball starts with the smallest balance, because quick wins keep you going. The avalanche starts with the highest rate, because that costs less. The difference can be calculated rather than argued about.
The avalanche comes out cheaper by 496,40 zł in interest. The snowball closes the first debt sooner, and that is the one thing the arithmetic does not measure: a plan abandoned in the third month saves nothing at all.
A fixed rate, contractual instalments paid every month, the spare money going entirely to one debt and rolling over -- together with that debt's instalment -- to the next once it closes. No fees and no insurance added to the instalment.
The avalanche always costs less, because it puts out the most expensive debt first. The snowball closes the first debt sooner, which is its only advantage but a real one: a plan abandoned in the third month saves nothing. The calculator shows the difference in money, so you can judge what the easier start is worth.
It is whatever you can add each month on top of the contractual instalments. It does not have to be large: once a debt closes, its instalment joins the spare amount and moves to the next one. Hence the name -- the snowball grows as it rolls.
That depends on the rates on both sides and on whether you have a buffer for emergencies. This calculator answers only the question of repayment order and does not weigh it against saving, which is a separate decision.
The arithmetic does, but a mortgage usually carries the lowest rate of all your debts, so both strategies leave it for last. For a mortgage on its own the useful question is the overpayment one: shorten the term or lower the instalment.