[HeavenFox]

In theory, you could use OFX, which is an open standard that powers the "Download transaction" feature of Quicken/MS Money

In practice, banks never tell you the address of their OFX server and you have to rely on community compiled database (eg ofxhome); many banks' implementations are iffy; some banks even charge you for enabling OFX support on your account. In the end it's just so much easier to outsource this to Plaid, which is why they are a billion dollar business.