The yahoo mail app requires iOS or iPadOS 12 or newer. If your iPhone and iPad are too old to be updated to iOS and iPadOS 12 then you won’t be able to use the yahoo mail app. Yahoo does not support older versions of their app and they are not available for download from the App Store.
In your device general settings, check update and see if your devices can be updated to iOS/iPadOS 12 or 14. If so update and download the app. If update in settings says your device is “up to date” and is running anything older than iOS 12 then your device is too old to be updated and you won’t be able to run the current supported version of Yahoo’s mail app.