How to remove the redirect to the "Thank you" page after placing an order?

Question

WordPress

After placing an order, the buyer will redirect to the success page, but according to the approved design, after placing the order, a pop-up with an inscription should pop up. I can’t find the form handler itself, the action on the form is on the order page, i.e. /checkout/ , but where is the form handler? Or how else can you turn off the redirect and add a popup?