You have it backwards, there is no separate installation of Safari for iOS or iPadOS, it only comes as part of an OS upgrade. Major (named) macOS updates come with a new version of Safari but it is also available to install on the two previous major versions of macOS. The current version, macOS Monterey (v12), came with Safari 15 and Safari 15 is also available for macOS Big Sur (v11) and macOS Catalina (v10.15).
The iPhone 6s was released in 2015 yet can run the latest version of iOS released in 2021. It probably won't be able to run the 2022 version of iOS but at that point it will have been seven years. I'm sure Android can't be updated for that long, I don't know if a current browser for Android can be run on a seven year old version of Android.
The latest version of Firefox (97) at least claims to run on Android 5.0 which is from 2014. It runs fine on my Retroid Pocket which is on Android 6. So at the very least that's available.
The iPhone 6s was released in 2015 yet can run the latest version of iOS released in 2021. It probably won't be able to run the 2022 version of iOS but at that point it will have been seven years. I'm sure Android can't be updated for that long, I don't know if a current browser for Android can be run on a seven year old version of Android.