To understand the creative better, refer to the PSD given below.
The following example displays the creative rendered on a mobile web browser.
Which PSD template to use?
As the ScrollX is a full screen format, it is best to design the ScrollX for a resolution of 375 x 667px. Also, as the designs are required to be double the resolution to cater to devices with higher ppi, the design file would actually be 750 x 1334 px.
Now when you access the template link, you will see that there are 3 PSD templates available. This is to help with the design in different scenarios.
Do keep in mind that in 90% of the scenarios, one design is sufficient as the creative when build appropriately, with layers, is responsive and will adapt to the different screen sizes.
Use the 320 x 480 (640 x 960 px) template when the sites where the ad is going to render tends to have a fixed header and a footer ad as well. Otherwise just use the 375 x 667 (750 x 1334 px).
It is also better to use the 375 x 667 (750 x 1334 px) as that's the most popular screen size and now screens are just getting larger from there. So, the difference between this design and the larger screen is less. Also, the design tends to adapt well for the larger screens as well.