brew install cocoapods

Yep. Just like that.

No need to bother with different installations of ruby, system one, or sudo-less one, or some other stuff that will confuse you by not working.

It includes that message by Flutter doctor:

CocoaPods is a package manager for iOS or macOS platform code.
Without CocoaPods, plugins will not work on iOS or macOS.
For more info, see https://flutter.dev/to/platform-plugins

see https://guides.cocoapods.org/using/getting-started.html#installation

This instruction confused me a couple of years ago. It confused me again half a year ago. It confused me this week. Every time it didn’t work. What the fuck.

Well, maaaaybe you should do this some other way if you are doing some advanced stuff, but if you just want to build that flutter app, this should be enough.