Apple sued Samsung for making earlier Galaxies look too much like iPhones, you know.
Seriously though, some aspects are possible. If the S5's default launcher doesn't support icon packs (I don't have one, but would guess not) there are many 3rd party launchers which do (Nova, Apex, etc). Install one of these, find an iOS style icon pack you like & apply that, then most of your app icons will have an iOS look. Remove all widgets, remove the app drawer button and just fill your screens with little launcher icons and folders and you have an iPhone-like desktop.
There's nothing you can do about the default apps. If you can find alternatives that are closer to what you want then you can install and use those instead. Obviously you can't run actual iOS apps (completely incompatible).
If you want deeper theming, such as changing the signal bars, you'll need to root and either find a ROM that let's you do this or edit the system files yourself (I personally think the iOS 7/8 signal bars are one of Jonny Ive's greatest mistakes, but just use that as an example of something that's possible only with root).
