Ask HN: How does Plaid work under the hood?

Does plaid just store your bank username/password that you provide it and then pretend that it is you logging in? I noticed that they still have to relay your 2FA codes to the bank, so I am pretty sure they don't have some special back-door API to access your financial data. I am wondering if (when they got started) Plaid used something similar to Selenium to basically scrape all these transactions. However, given that they have thousands of supported financial institutions (and website formats could change every day)... I am wondering how they keep things working....