Off the top of my head: going in-person to the bank, email, phone call or sms to a number that you previously informed to the bank (say when opening the account), otp a la authy or aegis. None of these require you to be on google or apple's walled garden.
Nor did GGP's approach require them to be in google or apple's walled garden.
That's exactly the point: there's an easy and common method that many people choose to use, but there is still a perfectly working method for people who choose to not use apple or google.
Banks have been closing up their in-person services as fewer and fewer people use them.
https://www.kiplinger.com/personal-finance/banking/is-your-l...
> Since 2020, the rate of bank branch closures in the U.S. has doubled. The majority of those closures come from large and very large banks, contributing to an overall 5.6% decline in total bank branches nationwide since the start of the pandemic.