For those unfamiliar with iDeal, it works largely like you'd expect an OAuth flow to work:

- you click pay now on the merchants website

- you get redirected to your banks website

- you login to your account, usually involves 2FA and whatnot

- you get shown the transaction details and you confirm

- you get redirected back to the merchant who gets a signed token proving you've paid

Over in the Netherlands it's pretty much an universal standard. All banks support it, and very nearly all webshops.