iPhone 5s, 6, 6 Plus design confusions around dimensions, resolutions and densities

I have to design an iOS app in iPhone 5S, 6 and 6 Plus in Adobe Illustrator. Can anyone help me with the ever confusing PPIs and retina/retina HD displays and dimensions of these?

For example, I first design for iPhone 5S which is retina display with 1136x640-pixel resolution. I use 72 ppi in Illustrator. Now since it's retina display, these pixel dimensions make it @2x, right?

Since now it's illustrator, I can directly export my artboard toother pixel resolutions/PPI to make the same design I created for 5S for 6 and 6 Plus. But, adding 36 PPI to 72 PPI thus making it 108 PPI doesn't actually give me the dimensions of iPhone 6 Plus.

I am thoroughly confused in this.

Can anyone help me with all of this with keeping in mind the following please:

I have to design for iPhone 5S, 6 and 6 Plus with Adobe Illustrator. I then have to give the assets at @2x and @3x to the developers