Interest won't be charged until the 'pay later' time is up. So, if you've chosen a 12-month BNPL plan, you won't have to pay any interest during the 12-month BNPL period.
If you don't pay back the full balance by the end of that time, you'll start paying interest on whatever's left. It'll be at your standard purchase rate, which you can find in the 'your account in detail' section of your statement.