I've learned the "Basics"; now what?

There's many things you can do.

When the player equips the 'iPhone 5s', you could use the clone() method to transit the gui to the player's PlayerGui

player = game.Players.LocalPlayer
gui = script.Parent.[guiname]

function onEquipped()
gui:clone() = player.PlayerGui
end

script.Parent.Equipped:connect(onEquipped)

Make sure you have both a script containing the above (to clone the gui into the player's playergui) and the gui, inside a tool (the iphone 5s)

With the gui, you can make animations similar to the phone's os.

Like if a player were to open an app, you could use these as an opening animation.

TweenPosition(UDim2.new(#, #, #, #), "Out", "Quad", nil, true)

the first set of parenthesis should have values for the destinated gui position.

TweenSize(UDim2.new(#, #, #, #), "Out", "Quad", nil, true)

the first set of parenthesis should have values for the destinated gui size.

TweenSizeandPosition(UDim2.new(#, #, #, #), UDim2.new(#, #, #, #), "Out", "Quad", nil, true)

first set of parenthesis should have the values for the destinated gui size. second set of parenthesis should have the values for the destinated gui position.

(you can change the nil to a time duration of how long the gui should Tween. higher = slower lower = faster)

Try not to forget the MouseButton1Click() event. That's important for GUIs, if you were to have buttons.

script.Parent.Button:MouseButton1Click:connect(function()
-- script what happens here when you click the button
end)

Other than these, you could check the roblox wiki for more help on the making of your iPhone 5s. Otherwise, you could ask questions here.