How to prevent bank account info getting hacked using iPhone

There is pretty much a 0% probability that someone got the information from your phone unless they had physical access to your phone and the required passcodes/passwords necessary to unlock it, open up your banking app, log into it, and read the information for themselves.

Use lengthy but easy to remember passcodes and use Face ID. Many banks support Face ID for their apps. Review the Apple Support article I linked below. Compromised accounts generally involve some kind of social engineering.

Recognize and avoid social engineering schemes including phishing messages, phony support calls, and other scams - Apple Support

See this excerpt from the iPhone user guide as well:

Use the built-in privacy and security protections of iPhone - Apple Support (SG)