bogwog parent
As a GrapheneOS user, the way I access my banking app is by downloading it from the Google Play store just like everyone else.
They don't all work, though: too many crank up the settings on google's various 'integrity' checks and will fail on anything that isn't 100% google-blessed. (Which is insane, because that's all that's required: on a previous phone of mine, it worked fine with a stock ROM with a bluetooth-based RCE, but upgrading to a custom ROM would have meant it was 'insecure')