~~privacyguides recommends Raivo OTP~~, see https://www.privacyguides.org/en/multi-factor-authentication/
Raivo OTP is a native, lightweight and secure time-based (TOTP) & counter-based (HOTP) password client for iOS. Raivo OTP offers optional iCloud backup & sync. Raivo OTP is also available for macOS in the form of a status bar application, however the Mac app does not work independently of the iOS app.
Its Github repo is at https://github.com/raivo-otp
EDIT: PrivacyGuides has removed Raivo from their recommendations due to it being bought by some company that people can’t find much info on. See this discussion https://discuss.privacyguides.net/t/remove-raivo-otp-ios-mfa/13305.
Currently people are considering different iOS options, for example here https://lemmy.one/post/1512945. I don’t see any consensus yet.