For most of them, interest accrues from the date of purchased but is not charged if you pay on time. If you don’t, you owe all the back interest from day one.
The parent poster is confusing BNPL with cobranded cards like Room 2 Go credit cards that have special promotions with “0% for x months”. It’s a regular credit card where the minimum payment won’t guarantee that you pay the original amount off in “x” months, but you only have to pay 2% of the balance. These have always shown up on your credit report.
They are “deferred interest” plans where if you don’t pay the balance off in $x months it will charge you back interest.