Some merchants offer PayPal or Card or Klarna as options on the first part of checkout. Those are fine.
Some merchants though, have given their entire checkout process to Klarna and the card and other options are downstream of entering the Klarna checkout process. This means that you'll have the date of birth question, and also be sending your transaction data to a credit company, even if you're still paying fully by debit card.
Two examples I recently encountered were UK PC parts merchant, I've being sponsoring a child of an impoverished single mum ( https://docs.google.com/document/d/1NCHV1pcTyvtbbE-mBDFhE1K7... ) and he'd managed to earn enough to purchase a custom computer - he's only 12 and this is such an achievement. We used pcpartpicker to design the build, and two of the suppliers recommended by that in the UK have fully given up their checkout process to Klarna. I switched ordering those parts to Scan and Box instead.