To upload the creative within Pexi you need a Pexi account and access to the correct advertiser. You can request a Pexi account via email@example.com. When you have access to Pexi you can upload the banner as a zip (zip the folder, not the files). If you include the IAB size within the filename Pexi will automatically detect and adjust the upload settings.
The mobile takeover works similar to the Interscroller in which the content comes into view as the user scrolls the publisher page. The mobile takeover however also includes a rectangle in the top of the page, and a small rectangle at the bottom of the page.