The app is written entirely in Swift and has a binary size of ~550KB. I avoided using any third-party library because the app can access your local iMessage SQLite DB. The app uses modern Swift features including SwiftUI and async/await. The app uses a custom, redux-like architecture (although the reducer is not currently side-effect free). The app could be written for older macOS versions, but that would have prevented using some of the newer SwiftUI features for menu bar apps.
The DEVELOPERS.md file includes some technical details on how to implement this from scratch, plus some links to related projects and useful blog posts. All in, there are just ~800 lines of code.
SIM Swapping is dangerous and far too easy. Use Google Auth or Authy or another TOTP at minimum, a hardware key like a Yubikey is your best option.
When I receive sms with the code, my mac suggest the code as autofill in the form.
