Worth noting it's opened up to regulated AISPs now too, so it's no longer only available for consumers to use to access their own information.
There's a waitlist over at https://monzo.com/business/ but you're unfortunately too late for the initial test of 100 users.
A partial list of banks that support OFX is here: https://wiki.gnucash.org/wiki/OFX_Direct_Connect_Bank_Settin....
For your own bank, look to see if they support import into Intuit Quicken or Microsoft Money. If they do, you can probably access the API directly yourself.
Note, you may have to pay a service fee to access the API. This will vary depending on the bank.
It’s an api service that integrates with most major banks.
Because what else can they do? They have the same access you do. Unless you don't give them it, which you shouldn't.