Unfortunately it doesn’t use any API, as is the case with most banks, Barclays don’t have an open API for 3rd party consumption.
Instead we read and iterate the HTML pages using regular expressions and form filling logic. It’s a very clunky, error prone method but unfortunately it’s all we have.
Maybe one day all banks will have free open APIs for read only statement access. At least the March Budget did allude to this prospect, although we will need to wait and see.